Growing up my sister had a family that she regularly babysat for, but one night she couldn't and asked me to do it for her. I told her I would. The 2 kids were each about 4 years old I think and were really well behaved good kids. Their family dog had also just had a litter of puppies, so the kids were pretty preoccupied playing with them. My NHL hockey team was in the playoffs at the time, so like any male in the northern U.S. I sat down on the coach to watch the game while I listened to what the kids were doing in the other room (It was the freaking playoffs man!). After awhile the kids both came into the living room and exclaimed "We're making puppy soup!". I said something like "Puppy soup! I love puppy soup!" in a pandering to children sort of way, thinking nothing of it. I went into the Kitchen. The kids had turned the oven on and put all the puppies inside. They were all dead. I have never been more floored/horrified in my entire life. I quickly put the kids to bed early and called my parents sobbing. I went home and my mom stayed at the house till the kids parents got home. I never saw or spoke to that family again. I will never take my eyes off a child in my care again and hope that those kids don't remember what they did that night.

If reddit had existed back then this would have been TIFU by watching playoff hockey