I was at a school camp in Primary School, and we had to go up onto a really high obsticle course. I froze at the very begining, when i had to cross a wobbly plank of wood. It didn't matter that i had a harness on, and that i was completely safe from falling, i just froze.
I keep remembering that sudden fear of hights, and i have hated it since. I sometimes have dreams about it, but not as many, thankfully!