Here's a couple.
First - the mail server is running out of disk space. We decide to implement the obvious policy of emptying everyone's trash, removing emails more than a month old. I immediately get calls from at least 5 users saying 'where have all my old emails gone?'. 'What do you mean' I say, 'you got the warnings that we're emptying the trash with emails older than a month?'. Every one of them then tells me that the trash folder is where they store _all_ their emails. The reasoning is that it's quicker to hit 'delete' than put them in a folder. They've got a point I suppose ...
Second - my first job at a top science and engineering university in the early 90s. Female professor of computing comes in one morning with a sorry-looking Sun 3/60 keyboard. 'It's not working', she says. I took a look at this rather greasy keyboard. After asking how it got this way, she told me... She'd puked on the keyboard. Thinking the acid in puke probably wasn't very good for it she decided to wash the keyboard. Not in water though as most people would do (and probably wreck the keyboard too but that's not important right now). No, she decided cooking oil would be a suitable cleaning solvent so she drenched the keyboard in it.