There are so many that it's really hard to choose, but I think this may be my favorite nonsensical dialog in Lotus Notes, our enterprise mail system of choice:
Some UI professionals would argue that it is not a good idea to gray out buttons. If selecting a button is not applicable but the user thinks of selecting it, then allowing them to gives the application an opportunity to explain why it is not appropriate (or would have been grayed out).
I agree, but I have a hard time thinking of any reasonable alternative. If an action doesn’t make sense, the user needs to know that… somehow.
If the thingamajig whatever (button, spin control, alien ship power setting slider in the flying saucer that just abducted you) is inappropriate for the modality of the moment, why even display it? I mean really, if the thing can’t do anything why even bother the user with the knowledge that it is there?
John: Speaking for myself, every user I’ve ever dealt with, including myself, hates the idea of "It just disappeared!"
Alan: Complete non-gray out makes it hard for a user to know if a button’s useful or not at any given moment. Would you really be happy with a menu where you couldn’t tell instantly if you could paste or not, and actually had to select paste, and DwtfD (Deal with the friendly Dialogue)? It would get old fast.