The situation where a recent catastrophe is further complicated by some application of Murphy's Law or further revelations. Taken from the mental image of the "fragrant and pictorial" mess that excrement would make if it came into contact with the blades of a fan...
It was bad enough when they caught him rolling in the hay with a girl, but the shit really hit the fan when it was reported that she was underage.
The toilet in the 1970's space station "Skylab" had a suction fan and a safety belt to strap yourself down with in zero gravity.
Zero gravity would cause the faeces to float all over the place disasterously so there was airflow to suck the faeces into a bag, a bit like a vacuum cleaner does. Now, if the bag broke somehow, then the "Shit hit the fan!!".
When something's bullshit, or literally shit, or when someone or something is being a pain in the ass or fucked up.
n. That was a huge shit from the asssssssssssssss
v. I'll call you back later, i'm gonna go shit from the asssssssssssssss.
adj. That test was so shit from the asssssssssssssss... i think i failed it.