"Net send" vs unprotected X11 defaults
I was on a co-op work term in uni, when I found out that X programs could as easily write on another display as one's own.
A fellow co-op student was parked in the server room of this Canada gov ministry's computer room, so I'd routinely send the nastiest pics to his HP workstation display. Usually strongly unattractive people, sans clothes. Especially due to the lack of privacy, he learned to keep a large window open that he could immediately bring to the front on his screen, then calmly deal with the offender once he was sure nobody was looking.