I remember seeing a prank program years ago that showed the SETI@home screensaver for a bit, then popped up an alert box saying "Alien Life Found!" with options to submit or cancel(!).

If you tried to submit it would spend a while with a really slow progress bar, and then say it failed to submit and asked you to contact SETI directly. I wonder if anyone actually did....

I wonder if that screensaver ever made it into the Xscreensaver BSOD screensaver. I hope so.