Mine were in the early 2000s. Back then, the computers at the lab at my uni were not very powerful, so people would do work at a Linux console, saving themselves the hassle of running a bulky X session.
Some time around 2001 I read the console_ioctl(4) manpage and found it replete with prank possibilities. I wrote little programs that would flip the console font so that all the characters were upside down; or swap capital letters with small letters, again by way of manipulating the font; or flash patterns on the keyboard LEDs; or fade to black and back by manipulating the palette.
I then added a server component so that I could leave it running at an innocuously-looking terminal, wait for a victim, fire up these effects remotely from another box in the same room, and watch what happens. Fortunately, I soon discovered that the coding part was more fun than the watching-people-slip part, so I gave up on the latter.
Another prank I used to do was simulate a successful root login on these terminals by just typing in what would be printed, including the motd, at the getty login prompt, simulating newlines with tabs/spaces (and never ever pressing RET), ending in `[root@mailhost root]# `. Then, again, step back and watch what happens. Some people would curiously type in `whoami` and be puzzled why they got a password prompt; some would step back in terror without touching anything, switch terminals and email the sysadmin.