I've been scared twice. First time, we were swapping critters around, and I went to move a sally ight foot crab. She was sitting in the back edge of the tank, so I very carefully scooped her up, feeling pleased that she wasn't running away--only to have it literally crumble in my hands as I brought my hand to the surface. Turns out I'd scooped up a recent shed, but it still scared the heck out of me.
Second time, we were pulling out everything to put in a sand bed. I picked up a medium sized rock that was convex on the bottom. I wrapped my fingers around the rock, and felt something prickly squirm out of the way. Freaked me out! flipped it over, and it was the large red and black brittle star pictured in the hitch hikers thread.
Oh, and I've had triggers do everything from spit and splash water at me, to bite me--that is a nasty surprise when you're not paying enough attention!