I often use applications and electronic devices for which I think: "Why on earth did they engineer that thing as it is? They must have known that it is a pain in the neck to work with".
On the other hand I often observed that I created a (G)UI that I was convinced about, that it'd delight my customers and was a breeze to work with. Although my customers thought that too, it became obvious that it wasn't at all easy to work with in day-to-day work.
Because of that I believe that there are many developers and designers out there who are genuinely convinced that their product has the perfect user interface, but it hasn't!
That's why I wrote this question: To collect some of the common misconceptions developers have about user interfaces and to prevent other developers (including me) from making the same mistakes.
What annoys you most in user interfaces of applications, web sites, electronic devices, etc. but presumably was created for the benefit of the user? What was it that you were convinced would be a great idea—but in the end only annoyed your customers?
Please write only one example per answer.