I used to have a cowfish in my tank, I had Bessy for about a year, she had never shown any interest in me what-so-ever when I would puit my arms in the tank to clean. One day I was cleaning and I see her come across the tank very slowly heading straight for my hand. Now, they do not move all that fast, or at least she didn't, and I had plenty of time to think "Hey, I think she is going to bite me." Well, she came up to my hand, found a spot she liked, and bit right in, she took a pretty good chunk out too. Of course, after that all I could think was, "D@mn, I cannot believe she bit me."
Some day I may actually start acting on these feelings I get.
Oh, and my cleaner wrasse loves to come up and clean my arm whenever I put it in, but that does not hurt.