Oh, you brought back some OS 7, 8, 9 memories. I had a startup item that would "melt" the screen into a puddle shortly after booting. Another would cause the mouse to bounce around the screen and become almost impossible to control. But my favorite was the one I put on my principal's computer. It would show all text backwards. When my principal opened his email he went crazy. He called in his secretary (I was lurking nearby) and said, "Somethings happened to my computer. It's in another language!."
After laughing til I ached, I fessed up and removed the hack. He forgave me eventually, then wanted to know how to put in on someone else's computer.