I had a GBR that would eat from my hand. I didn't intend to train him to do it, but whenever I would hold a cube of frozen food in the tank to just let a portion of it melt he'd nibble my fingers and the cube. Lol!
I'm looking after a betta, and if I held my hand under him he would sit in it. So, one time I was cleaning the filter and my hand was kinda cupped and he swam over to it, and of course I'm thinking "aw he's going to sit in my hand again"... Nope. He totally bit me