We banned a group of kids for throwing food at walls and just making an overall mess whenever they came in. My manager told them to leave, and not come back unless they were with parents, or she would call the police. An hour later, no, literally an hour later, they were back. So my manager calls the cops.
So generally when someone calls the cops on you, you learn not to fucking do that again. But they came back the next day, and were apparently suprised when my manager called the cops. Again. They still try. I guess 'banned' is too strange a concept for them.
As far as employee mistakes go, here's an interesting little story that could've ended terribly, but we never heard back from the guy:
So a guy comes through drive-thru and orders a filet. I'm presenting, so when I see a filet laying on the counter where they generally put food ready for drive-thru, I grab it, put it in a bag, and hand it out.
A few minutes later, my manager and a coworker are freaking out about where this burger is. I tell them that I handed it out. They then tell me that the particular filet I had handed out had been returned from a previous customer. Apparently, a woman had taken a bite, found something to be unsatisfactory, ripped it in two and just generally fucked around with it. So I had just given out a torn to shit fish filet to this poor guy. Suprisingly, we never got the angry phone call we expected. My manager just told my coworker that, perhaps, the waste bin might have been a better place for something like that.