Not a port a potty but I was in a grocery store bathroom once and evidently the wall of the stall acted as a public forum for all the male employees, I've never seen that amount of graffiti in a bathroom before or since.
At my middle school, someone drew an extremely crude picture of a stick figure getting his dick sucked by another stick figure and captioned it something like "I get my dick sucked by (our vice principal's name)".