Growing up in Middle school I remember creating a QBASIC program that was a full screen BSOD with a fake virus warning. I put it on a floppy (used an AOL floppy naturally) and loaded it in the autoexec.bat of various Win9x machines on display at Radio Shack, and Best Buy. This fake BSOD would pop up before Windows actually loaded. It was hilarious to watch the paid employees, at least 10+ years older than me, struggle with basic computer troubleshooting while we ate our ice cream at the food court across the way.
In High school I remember we could just disconnect the Ethernet, hit "Cancel" on the login prompt for Windows 98, and it would log in anyways, then plug in the cable to restore Internet access.
In university they had a pay credit system on the printers. I remember realizing that I could print to file (.PRN), then use netcat to send the file to the IP of the printer and bypass the print credit system. I created a BAT file to automate this (so I could select what printer in what computer lab). Once word of this got around, I started sending threatening messages to the LCD display on the printer "Print Theft detected, IT notified" to try and scare people off.
As a grown adult, when IT erroneously locks out users, on several occasions, I've pulled the Ethernet cord, logged in using the last known credentials for the user, then plugged in the Ethernet to restore Email, Internet, and local access.