When the shit hits the fan
Definition 1 of 1
Pro Tip 1/3Colloquial Usage
The phrase is informal and best used in casual conversation, not in formal settings like business meetings or academic papers. 
I knew when the shit hit the fan at the party, it would become legendary gossip.
Pro Tip 2/3Potential Offense
Be cautious as 'shit' is a swear word and the phrase might be considered vulgar or offensive by some. 
I was careful not to say 'when the shit hits the fan' in front of my grandparents.
Pro Tip 3/3Crisis Point
Use the phrase when a situation drastically deteriorates, and its negative impact becomes obvious to everyone. 
He kept ignoring the warnings until the shit hit the fan and he lost everything.