Two things, Ladders and large, tightly packed crowds of people. I don't have any fear of small spaces or heights in any other situation. I've stood on a ridge line as sharp as a knife, with a 100m drop to either side of me and been perfectly fine, and that same summer I went explring a local cave system, with gaps so small you actually had to let all the air out of your lungs to squeeze through, and never once did it bother me. But put me on a 1m ladder, and I'm holding on for dear life, put me in a room packed to capacity, and I start hyperventilating and frantically trying to find the nearest exit.
I can't really explain why these bother me so much, but they do, and it sucks somedays.