A common mistake of user interface design is to come up with a clever solution to a common user problem, when you should work out a way to remove that problem in the first place.
My example here is the password entry dialog, in particular the GNOME Screensaver one.
The common user problem is that their password is rejected, even when typed perfectly, because the Caps Lock key is on.
The clever solution was to display in the dialog that the Caps Lock key was on, and thus hinting to the user why it might have been rejected.
The better solution would be to ignore the state of Caps Lock in all password entry dialogs, so it doesn’t matter whether it’s on or off.