I received a phone call complaining that the PCs we'd set up at an event were not printing.
We'd installed twelve PCs in groups of three, sharing four printers - one installed upon one PC from each group, then shared out to the other two.
They were ranting and swearing, making threats, and generally being complete asses about it.
Being a 'bottle-it-all-up-and-go-psycho-in-your-own-time' kinda' guy, I remained as polite as possible. I asked a few questions and received unsatisfactory answers, so two of us jumped in the van and drove the 70 miles to the event, out of hours, to sort out the problem.
It turned out that (against our advice) they'd moved the PCs around, unplugging and re-plugging at random.
The reason they couldn't print was that none of the printers was attached to the correct PC; even the network cables had been incorrectly re-plugged (two even looping back to the same hub)!
I calmly explained this to the ignorant, foul-mouthed idiot who had phoned me, then handed him the bill.
His horror at the realisation that he would have to explain this extraordinary cost to his employer was enough to make my whole week - and I did not once have to lose my temper with him to make him feel about as small as his intellect.
Bonus!