Jakob Nielsen wrote an interesting article that explains some of the most common mistakes made when designing an application's user interface.

My favorite quote is:

The most common way to get usability wrong is to listen to what users say rather than actually watching what they do.

His Alertbox column is a must-read, if you do any work related to user interfaces (of either desktop applications or web sites/applications).

If only somebody could convince Mr. Nielsen to add a feed to his site...