Whilst working at OcUK for many years, I have seen many fails. But it happens to all of us right?
From someone using a glue gun, to make sure his SLI 580 GTXs were not going to come out of the PCI-E slot? And wondered why his PC was not working. - This is when the 580s were the top of the range cards back then...
Someone raging at me in the shop, saying that we have not fixed his PC because his PC doesn't even turn on anymore. Interrupting about 6 people waiting in a queue before him in anger, he plugged his PC in the shop to show me. I went around the shop counter to flick the PSU switch. He walked out quiet as a mouse after that and apologised.
And of course we sometimes get silly questions, like which way does the air flow on a fan.
I am sure I can list so many, but they are my favourite 3 that come to the top of my head.