Mom's Long Horned Cowfish will bite your hands when you start moving rocks around. It will definately suprise you. Scared the crap out of me the first time he did it to me. It didn't really hurt but I was not expecting it and I didn't even notice him eyeing me. It is more of a 'suction' with 60 grit sandpaper bite than a 'chomping' bite.
My yellow Tang lately has gotten into the habit of trying to 'lay down' on my arm or in my hands when they are in the tank. It is kinda weird but he seems to like it, but sometimes I have to push him away to do/see something and he thinks I am playing with him, he just comes right back and tries to push me, he has even nibbled on my knuckles a few times.