Brings back some fond memories of exploiting issues for fun and then reporting them before getting into trouble.
Discovering the lpr flag that (incorrectly) didn't check file permissions so you could print anything that you knew the path to without read access.
Discovering that the brand new Sun workstations and existing unix systems had overlapping userID numbers.
Escalating email auto-reply wars that filled the system storage.
Swapping around the serial cable connectors that were all jumbled up in the corner of the room and making people's sessions jump to a different terminal.