"When
the shit hits the fan" is the vulgar and more satisfying term for a situation that suddenly gets messy. It gives you an idea of just how messy a situation by
comparing it to being in a small room when a handful of loose feces are
launched into the spinning fan blades.
1: When that silly old bitch pranged John's prized Mercedes in the parking lot and then tried to take off, shit really
hit the fan. I though he was gonna kill her.
2: Where's our fucking money?! You better have it, or the shit is gonna
hit the fan, believe you me!!
3: I lost everything back in 2008 when
the shit hit the fan. Just like a lot of people.