Besides the creepy Gamestop guys I posted under another comment thread, I used to work at a movie rental store and people would flip their shit on the reg.
The most memorable was when a woman had a MUD (mixed up disc). She essentially returned a case with the wrong movie in it. By the time she came to me, the situation had been fixed but we still charged an (inconsequential, like, <$3) because hey, we didn't have the movie to rent out.
When I gave her the total with the fee, she demanded to know why she was being charged for the mistake. Honestly, if she hadn't been so rude I probably would have just taken the fee off for her right away and not worried about it, but I don't like rewarding shitty behavior. She wasn't belligerent just yet, just rude. So instead I just explained to her that we just charge the price of another night to make up for lost business.
This was an instance of bad timing, because if customers didn't come in for a few months we would sometimes give them late fee forgiveness. At the register next to me, my co-worker was doing one such transaction and taking off a $40 fee or so. The woman overheard, and started SCREAMING "I guess I should have gone to her! She'll take off $40 but you won't take off $3!" Then she stormed over to the other customer and said "Did I hear that right? Is she taking off $40 for you but this brat won't even take off $3?" The other customer just stared at her. Then, she turned to my co-worker and told her how I was giving her attitude.
At this point I told her I'd take the late fee off because I didn't want to deal with her anymore. She then accused me of giving her more attitude (to be fair, my annoyance was probably on my face. I'm bad at hiding that kind of stuff) and wanted to speak to the manager. My manager was literally right next to me at the next register, but was absorbed in a conversation and somehow didn't notice. I told her "Sure, as soon as she's done with this customer I'm sure she'll be happy to speak to you. Your total is $X.xx"
She threw a card at me. When I picked it up, it was a movie gift card to a theatre or some sort. So I said "I'm sorry, this seems to be a movie theater gift card. This is a movie rental store." So she got flustered and even more mad, and threw her credit card at me.
I moved away from the counter while she waited to speak to the manager. I made eye contact with the other customer she has accosted and he gave me a sorry smile, and I smiled and laughed a little back at him. That's about when my manager started to talk to her, and she went on about how I wasn't taking this seriously and was over in the corner laughing and how she shouldn't have such rude employees. I was still laughing at this point because she was being so ridiculous. My manager placated her and she left, and then asked me what happened.
My manager literally just went "Oh, okay" and left it at that.
Which is why complaining to the manager never works.