Seems more common than I thought
About 22/23 years ago, I worked for a large manufacturer. They were a Lotus Domino/Notes shop as well as big users of Citrix (the latter being the reason I was there).
One of the young lads on the helpdesk was a bit of a prankster and often got himself into a spot of bother for acting before thinking.
This one particular day, he was called out to one of the HR managers' for some problem on her desktop.
Whilst he was fiddling around with her computer, she went to get a coffee, leaving our intrepid plank alone for a few minutes.
Did I mention his dad also worked for the company at the same site. Not in am IT role, as I recall, but he had email.
He also had a sense of humour, so when he received an email from the HR manager that read "You're fired you bald bastard!" he knew it was a joke. He also had a good idea (he'd been on the receiving end of his son's pranks before) who had sent it, so he did nothing with it.
Again, though, the genius that the kid was, he didn't delete it from sent items.]
Yeah...you guessed it. The HR manager found the email and lodged a formal complaint. Due to his other pranks it ended up being a final written warning.
It was quite funny, though.