I have another!
My boss sacked me for "complaining about poor health and safety in the office" and replaced me with his wife. The truth was, she wanted to return to work after raising their kids and she fancied my job.
To get revenge, when I "cleared my desk", under supervision, I secretly kept the keys to the building. That evening I went to the office, let myself in, and removed a huge armful of some crucial, vital files, which I deposited in a huge wheelie bin a few streets away.
I then left the keys in a hidden corner of one of the desks (where they would eventually be found) and let myself out (they were not needed to leave the building or to secure it.)
I really enjoyed thinking of them tying themselves in knots trying to work out how I could possibly have taken such a batch of files without anyone seeing me.
The files, by the way, were all the details, including the attendee list and the record of payments, for several large, expensive seminars my firm was putting on, so losing them would have plunged them into a very serious crisis.
Served em right, though. Not ashamed at all.