Not normally a problem here...but for a short time period (a couple years or so) I could not walk on a log over any great height. It was not fear...it was something tied to the visual imput, my inner ear and my brain. I would lose all sense of balance.
I am not very clausicphobic(sp)...I can hang out in elevators, caves and deep into lava tubes, etc -- but the even the idea of being in a very narrow space where I can not turn around starts a panic response. I can even pinpoint the experience as a young boy that probably was the trigger for this phobia -- but the mind is a strange beast. Now, if one of my boys was stuck in a cave and I had to crawl through a narrow place to rescue him, I wonder if my parentual instincts would over power the phobia. Hope I don't have to give it that test!