The Truth, and Nothing But the Truth
I'm running a character that uses a non-PHB class, to wit the wu jen from Complete Arcane. One of the things about this class is that, every ~3 levels, you have to pick a "taboo", something your character isn't allowed to do lest she lose her spellcasting ability for a day.
My problem with this is that most of the example taboos are things that you don't, in D&D, really have to deal with. "Oh, by the way, I never cut my hair," you say, and voìla! your taboo for that level is fulfilled. Even stuff like "make a small sacrifice once a day" doesn't specify that you actually have to spend any money on it--heck, you could say that you prick your finger every day before memorizing spells, and that'd cover it.
So for Altariel's 3rd-level taboo, I picked one I'm actually going to have to roleplay: Cannot lie. (Note that this is in the sense of "cannot make an untrue statement", rather than "cannot allow someone to come to the wrong conclusion based on what I say", but still, it makes perfect sense.) If the universe is to be expected to listen to me when I say there's a fireball over there, I can't go around telling lies about other things, now can I?