1. A button that doesn’t work on mobile
This is one of those errors that can slip through even in a well-managed project – especially if the first tests focused on the desktop version and the mobile one was treated as “secondary” in the context of the defined target group. It may happen that the product team or the client mainly works on desktop computers, and the mobile version – although it exists – has not been tested across the full range of devices and scenarios.
The effect? On desktop everything works flawlessly. The contact form is sent, registration goes smoothly, the “Order now” button responds correctly. Meanwhile, when opening the app or website on a smartphone – suddenly the key button simply… doesn’t work. You click and nothing happens. Zero reaction. No message. No information that something went wrong. Such an error can result from a minor technical issue: an incorrectly defined clickable area, an invisible overlapping element, or a difference in the implementation of the button on mobile. Sometimes a single faulty line of code is enough to block the button on a specific device or browser.
Real-life example? A user opens the mobile version of a restaurant’s website, browses the menu, adds dishes to the cart, wants to place an order… and cannot, because the “Order” button doesn’t respond. They don’t look for the cause. They don’t call the restaurant. They just close the page and order from the competition. You lose a customer at the very moment they were ready to pay.
2. Payment integration without confirmation
Payment is the moment of greatest trust between the user and the system. It is also the moment of greatest uncertainty, because that’s when real money is flowing. And if something goes wrong at this critical moment – the entire relationship may collapse.
One of the most common problems in integrations with payment systems is the lack of a clear message about the transaction status. The user enters card details, confirms the transfer, waits… and nothing happens. The page doesn’t load further, no confirmation or summary screen appears. Did the payment go through? Were they charged? Should they try again?
As a result: some users pay a second time, assuming the previous attempt failed, generating additional strain on both the system and the support team, which has to manually process refunds for such “double” payments. Others contact customer service – causing extra workload for support. And some simply give up, leaving a full cart or unfinished transaction.
Interestingly, such a problem often doesn’t result from a payment failure itself, but from small gaps in handling “in-between” scenarios – e.g. improper processing of the response from the payment gateway, errors when redirecting back to the application, or failure to test less common paths. Such situations may also only appear under certain conditions: with a weak internet connection, on specific phone models, or after interrupting the process and resuming it.
Example? A user buys a ticket for an event. They go through the entire process, pay, but receive no feedback. No ticket. They don’t know whether to wait for a delayed delivery, simply refresh the page, or try again. They’re left with uncertainty and lack of trust – the worst combination when it comes to conversion.
3. Errors in communication with the user
Not every system failure has to be a red error screen. Sometimes the biggest problem is… silence. No information that something went wrong. No message telling the user: “we know what happened and we’ll help you through it.” A system that doesn’t inform about errors or provides incomprehensible messages leaves the user alone with failure. And most users – especially new ones – don’t have the patience to guess what they did wrong.
Example? A user placing an order fills in all the fields, clicks “Next” – and nothing happens. No transition to the next step or order confirmation, no error message or note about incorrectly filled fields. They don’t know that they put a space in the phone number or left one required field blank. The button doesn’t respond, the screen doesn’t change, and the user starts getting irritated. After a few attempts… they close the page. Sometimes the problem isn’t a code bug, but the lack of information that the user should correct something or try again. And sometimes the message exists – but is unclear, imprecise, or displayed in a place where no one will notice it.
The result? User discouragement, abandoned processes, and a negative image of the company.
4. Logic error – when the system doesn’t know the rules have changed
IT systems have one important trait: they are precise. They operate according to the rules and patterns coded into them. The problem begins when those rules change – and no one told the system.
The risk of such situations is especially high in fast-growing, changing systems like e-commerce, where complex discount policies or advanced product configurators often appear. In theory, everything works: an example configurator lets the user tailor the offer to their needs. In practice – if the logic doesn’t account for the broader context, it’s easy for the system to lose consistency, leaving the user confused.
Example? In an automotive store, a configurator was designed to customize motorcycle helmets. The user can select the shell type, graphic color, clasp type, additional sun visor, or intercom mount. Everything works perfectly – because the system was adapted to this one, specific category. Over time, the store owner decides to extend the configurator to other products – e.g. motorcycle bags and panniers, which also come in different versions, sizes, handle types, and materials. The configurator is assigned to this new category but doesn’t account for its specifics. As a result, the system starts to “fall apart”: incorrectly calculated prices, missing options, incorrect messages. This isn’t a programming bug – it’s the natural consequence of a system being used in a new context without adjusting its logic.
Such moments highlight the need to test the system in different contexts – including cases where its use may change. This helps catch potential consequences of extensions – before a frustrated customer does.
5. A permissions gap – when someone sees too much
In a well-designed system, each user should only have access to the data and functions they truly need. Sounds obvious? In practice – very often it’s here that dangerous oversights appear. These errors are especially risky, because they concern not only user comfort but primarily data security and compliance with regulations such as GDPR. Even a single unauthorized crossing of an access boundary – when a user sees or edits data they shouldn’t – can mean a data leak, an internal incident, or in extreme cases: an audit, a hefty fine, and loss of customer trust.
What’s more, such problems often don’t surface during testing of individual functions – because the functions themselves work. The problem lies in who they work for.
Example? A sales department employee logs into the client panel and – purely by accident – sees financial data from another department. They can download a file with other clients’ invoices, check order histories, and even change contact details. Technically everything works, but for someone who should never have such permissions.
6. Lack of preparation for increased traffic = spectacular failure
The system works great. Functional tests passed without problems, everything looks stable, but that’s because you tested the software… in lab conditions. With one or a few users. No pressure, no load, no real interest. Then comes a new marketing campaign. Banners, newsletters, influencers – everything clicks, people flock in. Within minutes, hundreds and then thousands of users enter your site. And then the problem begins: the site stops responding, forms don’t send data, users see error messages – and leave.
Example? A clothing brand releases a limited collection – the campaign went wide, and the interest is huge. Users log in, add items to the cart, try to pay… and the system can’t handle the load. Within minutes, the campaign that was supposed to be a success turns into a crisis. Customers are frustrated, products don’t sell, and support is overwhelmed with tickets.
Such errors result from lack of performance testing. Yet it’s predictable: after a promotional campaign, traffic will increase; we know when the season starts; big events naturally attract more clicks, logins, and searches. It can be simulated. You just have to remember – and commission load tests. Sometimes it’s not a bug, but a slowdown of a few seconds, that can decide whether a customer gets frustrated and abandons a purchase.
7. Order logic error – how to alienate a customer after “Buy”
There’s nothing more frustrating for a user than when everything is ready: they selected a product, added it to the cart, went through the forms, paid… and only then find out the purchase cannot be completed. Something went wrong, but no one told them earlier.
Such errors often stem from inconsistent system logic or lack of data synchronization. For example, the product has already been bought by someone else but is still shown as available. The system allows it to be added to the cart, purchased, and paid for – and only afterwards informs the user that the order must be canceled.
That’s why it’s crucial to check so-called edge cases – unusual scenarios where multiple users interact with the same data at once. Cart overload is simulated, stock limits tested, and the system checked to ensure it “reserves” a product early enough and, in extreme cases, rejects the order before the user invests emotionally – and financially.
Example? An electronics store. A customer “catches” a laptop in a great promotion – one of the last units. They add it to the cart, fill in the required data, and pay. Moments later, they get an email: “We’re sorry, the selected product is no longer available, your payment will be refunded within 5 business days.” From the store’s perspective – a technical error. From the customer’s perspective – wasted time and money, and above all: loss of trust. Worse, such situations are often hard to “undo.” Even if the refund goes smoothly, the user experience has already been destroyed – and it can’t be repaired with another apology email.
8. Navigation errors – a feature no one finds
Your system can have all the needed features: quick contact, product configurator, convenient filters, intuitive account management. But if the user doesn’t know they’re there or can’t find them – it’s as if they don’t exist at all.
It’s no joke – sometimes the biggest problem isn’t the lack of functionality, but its… hiding. Links leading nowhere, a button that looks like plain text, or a feature placed where no one expects it – just a few examples of such unconscious “hiding” of features. And when the user doesn’t find what they’re looking for, they start wandering, clicking randomly, experimenting – until they give up.
Example? A user enters a store’s page via an influencer’s link. They have a discount code and want to buy something. They add a product to the cart – but see nowhere to enter the code. It’s not in the order summary, not on the form, no “Have a promo code?” in sight. After a minute of frustration, they close the page – they were supposed to get a discount, but nothing suggests it. Maybe the promotion ended? Maybe they were misled? Whatever the reason – the sale didn’t happen.
Meanwhile, the code field is hidden… under a collapsible “+ Additional options” link, right above the “Order” button. Visible only to those who know to click there. Such errors aren’t “technical.” The system works. But from the user’s perspective – it doesn’t work as it should. And that’s enough for an otherwise positive experience to be wasted. The system must be intuitive not only for its creators and loyal users, but also for someone using it for the first time.
9. User interface (UI) errors
A modern system is not just functionality – it’s also the look and user experience. An aesthetic, consistent graphic interface can build trust, make navigation easier, and simply make the user enjoy using it. Unfortunately, an interface that looks great on one device won’t always work equally well in other conditions. Resolution changes, screen size, browser type, phone orientation – and suddenly the perfect layout starts to “fall apart.”
Example? In the mobile version, two buttons – “Save” and “Cancel” – are placed too close together. The user finishes filling out an important form, clicks the wrong button, and the data disappears. No warning. No undo option. All because on a small screen the clickable area is too small or too tight.
These aren’t errors from negligence – but from the variety of environments modern software must handle. That’s why it’s essential to check how the system behaves in different conditions – on older smartphones and tablets, in dark mode, with enlarged text, in portrait and landscape, with the on-screen keyboard active. The interface must be not only aesthetic but above all functional, responsive, and accessible – no matter what device the user uses. Even the nicest design won’t fulfill its role if it’s too hard to use.
10. A feature works differently than the user expects
There are situations where everything works according to the documentation, without any errors. And yet – the user ends the session frustrated. Because it’s not just about whether something works. What matters is how it works, and whether it works in line with the user’s expectations.
This type of error is particularly insidious because there’s no obvious failure. No error notification, no system crash – but something still feels wrong.
Example? A form that the user fills out for several minutes contains a “Save and continue” button. Sounds like continuation – but after clicking, the user lands on a “Thank you, completed” screen. The data was saved, but the process ended. The user is surprised, confused, sometimes even cheated.
Another situation? You fill out a form, accidentally go back one step – and all the entered text disappears without any message. No “Are you sure you want to exit?”, no draft save option, no auto-save. Everything is suddenly gone.
These things frustrate the most – they aren’t obvious bugs, but they are user experience errors. They work against the logic users are accustomed to, or expect, based on system messages. And if something isn’t intuitive – it works wrong.
Better to prevent than to fix
We admit – we scared you a bit, but for a reason. All the errors described above are real examples of situations that can happen even in well-run projects. Because creating software is not just writing code. It’s a complex process that also includes planning, testing, understanding users, and predicting what may go wrong. And it’s precisely at this stage that it’s easiest to gain an advantage – before the product even reaches customers.
It’s not that every project is doomed to fail if you don’t foresee every possible scenario. But it’s worth realizing that many problems can be caught earlier – before they turn into costly fixes, unplanned downtime, or negative user reviews.
The role of the tester in this process is crucial. They’re not just the last line of defense against bugs. They’re a partner who brings another perspective – the viewpoint of a real user and the experience of catching situations that at first glance seem harmless.
If you care about a system that not only meets technical requirements but is also stable, secure, and effectively supports your business – don’t skip testing. It’s not a cost – it’s an investment in quality, reputation, and above all, peace of mind.
And if you’re looking for a team that really knows this stuff – write to us! We’ll help you create a solution that not only works but also makes a great impression.
