The toilet stalls at the University of Oslo had black doors and walls, possibly to avoid graffiti, but more likely a result of 1960s Scandinavian Design. The result was that the half inch or so of pinewood edging around the doors was crammed with mini-graffiti. One I liked was similar to Gnu's, located way down on one side of the door: "You are now shitting at a forty five degree angle".
Another from the same place (Frederikke for those who've been to the UiO) in 3 different handwritings. The first two are well known, but the third one I've only seen that one place:
Some come here to sit and think
I come here to shit and stink.
Others come to scratch their balls
And read what's written on the walls.
When all those other folks are gone
I sit here and whack my dong!