Office Pranks
Sorry About the Dent

Get your victim second guessing an imaginary card dent.
Step 1: Write a note that apologizes to your victim - saying that you are sorry about the dent in the car and that your insurance will pay whatever expenses there may be. Don't sign it or put any identifiable information on it.

Step 2: Stick the note somewhere on your victim's car where they will see it (probably the windshield or driver's side window).

Result: When the victim reads the note, they will start searching around the car looking for the dent. As time goes on they will likely get more frantic about finding the imaginary dent.

To make things better, you can come in the next day at work and confront your victim and say "Hey, I saw somebody dented your car... that sucks." It will really confuse your victim.