In high school in 1988, a friend and I discovered a vulnerability in the netware deployment of 30 IBM PS/2 Model 30-286s in our brand new computer lab that allowed us to insert programs into the autoexec netboot sequence. Prior to that, we'd been hacking on the (brand new at the time) VGA registers and figured out how to switch from 80x25 text mode to 320x200 256-color graphics mode with no flicker or glitches, as both modes have a refresh rate of 70Hz. So he created a TSR that preloaded a digitized picture of a clown face (a popular upload on BBSes at the time) into A000:0000, and after about 4 minutes, it would display the clown face for a few frames and then immediately switch back to whatever the user was working on. We gave ourselves away because we couldn't stop laughing in the corner of the room watching all the students get very confused/horrified looks. One bonus was the comic timing of a student calling the instructor over, having him stare at the screen for 3+ minutes, turn away, then have the clown face flash when he wasn't looking.

My friend's name was Brian, one of the smartest people I've had the privilege of knowing. Ten years later, we created mobygames.com.