Seeing as I'm going on ten years in retail I've had quite a few terrible customers in my day. The worst are when you're working the return desk because everyone will universally hate your return policy.

One of the angriest customers I had I actually felt for because he had a legitimate reason to be upset. This was when I was working for a one hour photo lab (back when people still used film in their cameras) and some guy came in with a roll of special black and white film to be developed; it was full of pictures he'd just taken of his newborn baby. Well he just so HAPPENED to come in when I was on break and had a less knowledgable coworker covering for me, and it just so happened that this coworker didn't know we weren't equipped to process that type of film. Queue the guy's film getting completely destroyed and losing those pictures forever when my coworker put it through our machine. I felt bad when I was telling the customer there was nothing we could do, but, well... there was nothing we could do. Bad timing.

Anyways I now work at 4am specifically to avoid customers because they are terrible. I'd rather work a ridiculous shift than deal with people.