We did this to a guy back in high school. We were playing football and he had been being a doosh all day and pissing us off. So when he went into the porto-john, we tipped it...
onto the door.
It wasn't that full and the water didn't shoot out of it that bad. He was screaming for help but we didn't and he had to self tip it onto another side so he could get the door open. We could hear the nasty water sloshing all over and he was screaming as the sh*t water was splashing on him.
We waited until he got out before we laughed at him and drove off.