Highschool: Maths teacher was sent some data on a 5.25" disk, with the instructions to remove it from its cover, and slot it into the drive on the computer.
The person writing the instructions called the cardboard sleeve the cover. The Maths teacher misunderstood, slice open the protective plastic cover, removed the disk, and placed that into the drive. It worked, but at the age of 15, was my first experience of PEBKAC.
10 years later as a field tech, get called out on a "now" response call, where the head beancounters PC won't switch on. $750 just for me to walk through the door.
This is back in the old AT psu days, where they head a pass-through connector for the monitor to be plugged into.
Beancounter was in the habit of just switching off the PC, which in turn powered off the monitor. Beancounter goes off on holidays, temp beancounter comes in to do the books, and was in the habit of switching off both monitor and PC.
I'm sure you can work out how I "fixed" the problem.