For those who work with the public: What was the worst situation that occurred between you and a customer?

So, so many; I have spent far too long in hospitality.

A relatively benign one: At a small coffeeshop. Found a quirky little hat behind the counter - a tweed fedora thing with a rakish orange feather in it. Full of youthful high spirits and caffeine, I proceeded to lark around the shop in the hat (yes I wish I still had that much energy), much to the amusement of my coworkers. A lady walks in, interrupting us. She'd left her hat.

At a bar: A nonzero amount of customers buying a bottle of beer and then pouring it into the tip jar (edited to add: thinking it a glass for their beer). Some version of "how was I supposed to know?!", repeatedly. I don't know, maybe the fact it was covered in chalk pen, contained money, and was very, very ugly?

At a pub: boomer getting very angry at me because he'd asked for a spiced rum and coke and I'd offered Sailor Jerry's (cheapest). Apparently that was a wussy drink. No, he clarified, a girly drink. He chose some other rum with coke. Yeah drink your sugar water old man.