I think my dumbest customer story isn't programming-related but still related to computers. I worked in a small computer repair shop about 3000 years ago, and one day a customer comes in with their family computer that's "not working." It turned out to be full of viruses and malware, and when we started working on it it turned out this was due to somebody visiting shady porn sites and clicking on download buttons left and right. I explain the situation to her and then recommend steps on how to avoid this happening in the future, so how to browse safely, antivirus software etc. She feelt a bit embarrassed and says that it's her son, and that she'll give him a talking-to.

A few weeks later the same customer comes back with the family computer and this time she's visibly annoyed, and curiously she's brought along the keyboard, mouse and monitor. The computer's got viruses again, and it's my fault. Why? Because she'd had a talk with her son who had then sworn up and down that he'll mend his filthy ways. When new viruses cropped up, his explanation was that obviously they're in the keyboard, mouse and monitor too, and since they hadn't been in the shop they were still infected and we were just too incompetent to have known this. Naturally she believed her son over my word, and started demanding that we remove the viruses from all the peripherals. I tried for a very long time to explain that it's just not possible (this was a time when PS/2 connectors were still pretty common and that's what they had so it wasn't even theoretically possible), but she wouldn't budge because her son was a computer whiz (he wasn't) and a Good Boy™ and would never lie, so clearly I was either incompetent or lying.

Finally I just relented and said "OK you got me, it's possible your viruses came from the peripherals but I just didn't want to mention it because removing them is so time-consuming and difficult". I took all their hardware in and had it unfucked in pretty short order, and I looked at the browser history to make sure that it really was a reinfection via the web, which it was (I remember Pamela Anderson featuring in a lot of the searches, which we techs giggled at.)

I kept their hardware at the shop for a couple of weeks; it's a tricky and demanding job to remove viruses from mouses, keyboards and monitors, remember? When writing the bill I charged her double the time I actually put in – she didn't want to pay at all because she felt it was our mistake but at that point my boss, who was a formidable lady, practically put her boot up the customer's ass and made her cough up the money.

She left in a huff never to be seen again, thank the gods.