In general, I'm not someone who craves absolute consistency in my game rules. In fact, I rather like it when there are rules that either defy expectations or don't make much immediate sense. A good example of this is the cleric's acquisition of spells in OD&D. According to the LBBs (as well as Swords & Wizardry, ACKS, and Labyrinth Lord Original Edition Characters), clerics don't get any spells at first level at all, but, at sixth level acquires both third and fourth-level spells for the first time. This is a quirky little rule that AD&D (and its successors) abandoned when it smoothed out some of the game's rough edges, but I've come to like it a great deal.
So, today's question is this: what odd little rules in your favorite RPG have you come to love and embrace wholeheartedly?