Inspired by the secret thread, which reminded me of this.
When I was about five I drew a house in biro on my parents' single piece of antique furniture. I have no idea why I did this. I was generally an extremely obedient, rule-abiding child, the sort that got nervous when other children were acting out. I was just suddenly overcome with an irresistble urge to draw the house. I still remember the feeling, and I don't know why I did it.
Did you ever do something inexplicable and out of character, that you still don't understand?