Well, I never did. I was much too scared to.
However, one Christmas morning as my mother, brother and I were opening presents, my kid brother, who was in 3rd or 4th grade, would shake a present before opening it and proclaim, "I believe this is a video game, it feels like Jurassic Park II." My mother was livid, as he kept doing this with all of his gifts. Turns out she'd left a bunch of receipts out and he saw them.
DH was always threatened as a child that if he went snooping, Santa would know and he would have nothing to open on Christmas morning. Well, his parents sent him off to do some chore and he accidentally saw a bunch of hidden, unwrapped gifts. He was sooooo upset, hysterical and crying. His parents tried their best to reassure and console him that everything would be fine, but he was very upset. He still talks about that Christmas as being one of the most traumatic events of his childhood.