I confiscated a Harry Potter book.
In my defense, I didn't know thats what it was when I snatched it. The kid (Shine is his name. Seriously.) had it hidden under his text book while I was trying to teach. It even took me a while to confiscate it because I used to do exactly the same thing in school. I would read a book under a desk or something so the teacher wouldn't see. It caused a lot of embarrassing moments, but obviously not embarrassing enough for me to stop. Finally, my annoyance at this kid - who was one of my worst students, overrode my camaraderie with my fellow reader. I walked up behind him and snatched it.
I scolded him for reading in class, told him to do his work (all the while carefully keeping his page - I'm not that mean) and went to sit down. Only then did I notice that I was holding The Goblet of Fire in my hand. I stared at it for several seconds all the while screaming in my brain "What have I done?"
I already started with the rationalizations: I'm a teacher, he needs to be paying attention, he can read it any other time, etc. Has no effect. I still feel terrible. I tried to play it off. Pretended to read it out loud when clearly I had no idea (it was in Korean). They all laughed, even the kid I took it from so I guess it was okay... Still, I'm always going to check what I'm confiscating from now on to avoid this in the future.