I love mountaineering/rock climbing but am terrified of heights. As in "proper paranoid and being unable to move another centimeter"-terrified, when it hits me. Being roped up usually holds it at bay though. Unless my belayer shouts down "Hun, how do you do the munter's hitch again?" at the second pitch*. And yes that has happened, and it was one of the worst nights I've ever experienced, as my subsequent panic attack slowed us down enough that we ended up descending an entire scottish mountain shuffling down on our bums for fear of falling over a cliff (the same guy didn't think it neccessary to take a head torch, as "we'll be well off the mountain before dark").
My biggest phobia are spiral staircases, though.
*For the non-climbers amongst you: A munter's hitch is a special knot which is used to bring friction into a rope system in order to arrest or slow down a fall. If you do it wrong there's no friction and the belayer wouldn't have a chance holding you in the event of a fall.