Is the app you’re developing supposed to be used by humans? Then you should do usability testing! User interfaces have errors just like any other part of IT systems. You test your code, but are you testing the user interface? Are you sure you’re getting the right input? Are you sure that what you’re outputting to the screen is understood by the human?
The best way to uncover errors in the user interface is to observe actual users try to use the system while you observe. A simple setup will enable you to do this more often. All you really need is a room, some tasks, something to test and a few potential users.
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