Convince newly hired employees that a certain employee, we'll pretend his name is Bill, has a wooden leg. Try something like this:

You know, not many people know this, but Bill over in accounting has a wooden leg. I just thought you should know. Now don't tell him I told you because he denies it to everyone, but he gets really pissed off when someone asks him.

The fun part is when you can get a fellow employee to help corroborate.

"Hey man, don't be spreading rumors about Bill! He really hates that!"

Invariably, the new hire will have to know the truth and will go up to Bill and mention somehow bring up the wooden leg.

Currently this prank has been pulled for the last 10 years at a bar the employees at Planet Wally tend to congregate. Every waitress is told about Robert's wooden leg by another one of the employees which sends Robert right over the edge.

This prank works with any place that has a large group of people where not everyone knows everyone else.