Here is one of the finest pranks I have ever heard of. From the Dilbert Newsletter:

A co-worker of mine has SETI@home running on his computer. This is
software, distributed by SETI (Search for Extra-Terrestrial
Intelligence), that will run on PCs as a screen saver and analyze
chunks of data from a radio telescope looking for non-naturally
occurring signals from outer space. The other day I copied the SETI
analysis screen to Microsoft Paint and then edited it to contain a
large alert message stating that ET signals had been discovered.  I
also drew in a button that he could use to "Notify SETI
Immediately." I left this image on his screen with a "red alert"
sound running in the background.

When he returned to his desk he was ecstatic to see that he had
found ET life.  He called another co-worker over to witness the
historic moment. Then he clicked the button and discovered what I'd
done.