I underestimated the size of a rock wall and free climbed it, only to lose my nerve about 3/4ths of the way up. So I was just hanging there freaking out, thinking I was gonna die. I managed to climb up and I've been scared of heights ever since.
Don't get me wrong, this wall wasn't THAT big, (40 feet or so, from what I can remember) but it was still scary as hell.