When you take a dump in a bathroom that does not belong to you and you neglect to flush, leaving the evidence for someone else to view upon entering the next day/week/etc. Also known as a dump-and-run, random shitting, mystery shitter syndrome (MSS).
After a night of drinking and partying:
Rod: Guys, I really need to shit. Do you mind if I use your shitter?
Ric: No problem.
The next day upon entry into the bathroom:
Ric: Holy Shit, I've been totally Schulered by Rod. What an ass!