I can't say I was involved in this prank directly but a co-worker of mine was. Some years ago he worked in a company that had hired 2 contractors to finish some programming work. The 3 of them worked pretty closely together as a team for 6 months. On the last week before the contract was due to finish one of the contractors, lets call him Dave, left his computer logged in when he went to get some lunch. The other contractor, Fred we will call him, was away in a meeting at the time. My friend spotted an opportunity and pounced. He wrote an email on Dave's laptop to Fred that went roughly like this. "I love you, I cant face not seeing you again after this week. Reply if you feel the same, if you don't I will understand, Love Dave." My friend said there was no visible reaction at the time from Fred. They both left at the end of the week. He met Dave a few years later and asked how Fred was. "No idea, he left our company soon after your contract ended without saying a word to me"
That's a big one. Did he tell Dave about what he'd done? What if Fred went on to work on cool projects that he otherwise might have included Dave on etc? That's the kind of prank that a week later you fess up to, you don't wait a few years.