At least you haven't wiped your hard drive without backing up. Which reminds me, its time for to backup....
Anyway, I'm a dill. The most recent blunder was that I was talking to some-one and thinking to myself, I know this person but I can't remember who it is. Three seconds later I turn to a friend and make conversation, saying "Where's so and so, your flatmate? I haven't seen her in ages." My friend replied, "She's behind you. You were just talking to her."