The comet shit can be defined as a large hard turd (sometimes nearly perfectly round like that of a donut hole) followed by a spray of loosediarrhea. When defecating, often times the thrust necessary to push this initial round turd "out" creates a rocketing ball of feces followed by a shower of watery slime.
A combination of shit hits the fan and worse comes to worst.
An expression used to convey that a situation has taken on a negative path or direction, or the situation has evolved into the absolute worst possible, chaotic state.
Joey thought it would be funny to take Benadryl and laxatives at the same time just before sky diving. Little did he know, this would affect his ability to pull his chute, therefore causing him to become a Shit Comet.