Simpler but funny tech support anecdote: at some point, someone called me over and said their keyboard would print the letter 'e' repeatedly. I tried another keyboard of course, and it still did it! Rebooted the computer, still 'e's all over.

Finally, I looked at her mouse: it was half of a wireless mouse-keyboard combo. Turns out, the keyboard hadn't been turned off before it went in the pile, and other keyboards on top of it were pressing its keys, or rather just the one key.