Principle of least astonishment
Funnily enough, I was just thinking about this yesterday when I noticed that on Facebook, when typing a comment or making a status update, you type @ and then start to type a friend's name, the top suggestion (and the one you will get if you hit enter) can be someone not on your friends list.
If you have just one person on your friends list whose name starts with the first few letters you've typed after the @ symbol, is it likely that a) I am wanting to tag them or b) I'm searching for someone other than them?
Hmmm. Facebook seem to think that is a tough one to call. :o)
Same thing goes for what Alistair is talking about - whilst it's true that it is good to push boundaries and innovate, baffling the user is almost always a bad thing.