Replace /dev/null with a regular file. Then laugh, waiting for people to fill up their bit buckets. (cc @hal_pomeranz , who is a clever and devious fellow)

How about removing the system's default initrd.img? Overwriting the boot sector? Much nastier than just filling up the root file system.

Dump /dev/random to standard out every now and then.

Remember “cat /dev/random > /dev/audio” on Solaris? Great for locating machines in your data center and annoying fellow Sys Admins!

I'm still proud and impressed with @kesterb22 sending audio over SSH to rickroll me and @defreez. And once I ejected a remote CD drive tray into somebody's head. Hilarity ensued.

The fun days when X11 was allowed freely between machines and you could pop 'xeyes' on your friend's display while they weren't looking