Extreme heights....like when I was standing at the edge of the drop-off at Mount Whitney, or a couple of the really narrow (2-3 feet wide) places on the trail to the summit....

2,000 feet straight down, right next to your foot. You won't even bounce on the way down, there's nothing in the way of free fall.

yup...don't do so good with that.

Also, falling off my boat when I'm sailing solo is a biggie. That's not a freak-out, that fear just inspires a mighty healthy respect for hanging on, wearing a safety harness and paying attention to what I'm doing.