In high school I was fired from a bagel shop because one of the OTHER employees was caught stealing. They fired that kid and put new rules in place, one of them being that if you weren't on the clock, you couldn't go behind the counter.
So a week or so later, I literally forget about the new rule, come to the shop, go behind the counter and make my own sandwich--the kids working that day tattled on me and the next time I came to work they fired me! As an adult, I totally get their frustration with brainless teenagers, but I do think I should have gotten a warning because I was otherwise perfect and it's expensive and annoying to retrain someone new. I was such a goody two-shoes and to get fired...omg I was mortified.