When your ass turns into Mt. St. Helens and spews molten rock and lava from your asshole. Depending on what you ate beforehand, the pain endured can range from a mild annoyance to the equivalent of a soldering iron shoved deep into your rectum.
(Person 1 hears violent screaming emitting from the bathroom door)
Person 1: Is someone getting murdered in there?
Person 2: Nah, it's just Jim. He got diarrhea from that Taco Bell he had earlier.
Person 1: Ah.. Poor Jim. :l