Since you brought up UI bugs....

My favorite UI bug is in (now) ancient versions of Interleaf (a desktop publishing application -- think Framemaker or Quark Xpress) for Unix. You may or may not know that most Unix machines' mice have three buttons. To do most anything in Interleaf required navigating this remarkably deep set of pop-up menus. Most tasks required navigating at least 4 levels deep, but I've gone up to 20 or so levels. You may think that this is my favorite bug, but it's not.

Once you got to the item you wanted to select, you had to press the middle button. If you forgot and accidentally pressed one of the other buttons, you got a beep and a big popup modal dialog that said ``You need to use the middle mouse button!'' (or something like that -- I'm positive that it used an exclamation mark).

I love the fact that despite the fact that it obviously knew what I wanted it to do, it chose to berate me and make me navigate through that menu structure again. I hated that program.