Tell us of your great practical jokes. Here's mine:

background:
1) my supervisor was an ass (long gone now). Always backstabbing, looking to get someone fired.

2) we had a daily newsletter that came over the computer. this was long before the days (at this company, anyway) where such things would come as an email or webpage. It was kinda like teletype, but it was on the screen. It was like a DOS screen, and you would type in a code and this newsletter would come up on the DOS screen.

joke:
it so happened that after you called the newsletter up you could hold the backspace key and erase everything and type what you wanted. I wrote (in very official looking language) that the base we worked at was closing and everyone working there was going to be layed off the next week. I then called the supervisor over and said in a somber voice "read that."

He read it, got a shocked look on his face, and ran upstairs. Ten minutes later he came down with the manager. By this time I had already closed the screen so he had to call it back up, which would give him the original version. They called it up, and I watched as confusion ran over his face. He turned, looking for me, and as he saw me his face and neck turned bright red. I started laughing and turned away.