I have a couple.
I worked at a pizza hut in a bad area a few years ago. It was a smaller one, didn't have a full restaurant, just two tables. It was mostly carry out. For that reason people usually just came in to use the bathroom. We never thought much of it.
Until one day when one woman went in, and stayed there for quite a long time. eventually she started talking through the door about how she's going to kill herself in there. We called the police but before they arrived she staggered out, and started standing in the middle of the road. We did figure out her issue though, we found a used needle in the bathroom. We made the restroom customers only after that.
I also worked at a cash for gold place [Which to date is the worst job I've ever had.] It wasn't uncommon for stolen things to come to us, but it was my first time dealing with such an issue, my manager was there and told me to just finish the transaction before the police arrived. I felt really wrong doing that, so I took my sweet time. And before I knew it the police were there, the guy trying to sell to us had a habit of performing home invasions. I guess he thought he'd get away, because he broke off a piece of the plastic sign and tried to stab the cop with it. I'm pretty sure he was strung out because he missed the cop entirely and ended up with assault charges too. My manager got mad at me for not completing the transaction, and I left the job shortly after that.
My last few come from my last job. I worked at a call center for a certain phone company. But the stories here have less to do with the customers, and more to do with my co workers.
My first story has to do with a really unique smoke break we had one day. I was still in training, so we frequently got breaks, I always took the chance to smoke since I don't really do well around phones, and I was stressed. One of the girls who were also in training announces literally out of nowhere "I got my nipples pierced." excitedly, the majority of us ignored her, but there was another woman who joined her, asking to see. Next thing I know we're all getting flashed by this random chick. I guess she didn't know the smoking area had security cameras... She lost her job and got arrested for indecent exposure.
Another woman was nicknamed "flea lady" may god have mercy on your soul if you work near her. I'm sure not everyone noticed, but I'm allergic to flea bites. It was a fucking nightmare. Company never did anything about it though.
The most terrifying memory I have of working there would have to be the day the woman with severe anger issues got fired. To put a long story short, she destroyed some of the computer systems, actually assaulted coworkers that she disliked, and had to be wrestled down by security before the police took her away. I was pretty lucky, My cubicle was on the other side of the room, I was entirely out of her path.
I have a ton more, but I'm sure I've gone on long enough