First of all, before you do this prank, please be advised that some work places do not appreciate such conduct. I do know, however, that some places of work do not mind such activity, and I certainly hope that your place of work is one of the latter.
For this prank you'll simply need:
2 toothpicks
1 jar of mayonnaise
Go to the desk of the individual you wish to prank and place the mayonnaise jar to the right of the keyboard. Now, tape the two toothpicks to the lateral sides of the keyboard, and turn the monitor so that it's about 45 degrees right of where it would normally be. Now, when the person returns to their desk, it will appear to the office that they are moving in a sort of curved fashion as opposed to orthodox.