I used to work in a small office of 6 people which was a branch of a larger firm.
One April Fool's Day I set up an elaborate prank, faxing in that our office was being closed down due to the incompetence of everyone involved in our operation.
I used offical stationary, faked the owner's signature and sent it from the neighbouring unit.
The place went crazy while I watched each reaction, after about 15 minutes I shouted out, "April Fool!".
I'm glad everyone had a good laugh, don't do this if anyone is a heart attack candidate.