One of the funniest pranks we've seen is to put a very small piece of scotch tape over the laser on your marks mouse. The mouse wont operate very well! It is very hard to catch this too if you just use a small itty bitty piece of tape. Once we did this to a fellow worker and he got so pissed off he slammed the mouse on his desk over and over until it broke!

You can also do things with a ball mouse like tape the little spinning wheels inside the mouse so it wont register movement. Good stuff!