Too many to mention them all....
When I worked for a Politically Correct Planet in the 90's a guy bought a new printer but to save money he would use his existing printer cable.
Get it home faulty printer, bring it back works fine, home, fails, back at the shop works fine. He even brought his computer in .. worked fine.
He eventually brought everything in, including the oldest rattiest printer cable I'd ever seen. Of course it wasn't bi directional so didn't work with the printer.. I think he was given a cable in the end. All that to save a fiver...
Then there is the guy who held the VGA cable up to his computer and decided it wouldn't fit (he hadn't tried it it was just "obviously wrong") so phoned in saying he had the wrong monitor. After making sure that no pins were bent, he was trying the right place (colour coded cables what a boon!) it all sounded ok, so I asked him again to try it.. after many minutes of back and forth to try it, he eventually tried it.
He said he had been silly and remembering that the customer is always right I agreed with him.
Some one bought a new Compaq computer and it had a "faulty" power supply. I (now a service engineer) drove an hour or so to his house with a new power supply, looking like all the world a prop from Ghost busters.
I went in pulled down the flap at the front and pressed the power button. it sprang to life. a few more tests, all working, I asked it was an intermittent problem. he said he'd never seen the flap and had not known where the power button was....