When something goes terribly wrong and makes a gigantic mess that, typically, one individual is left to clean up. As parents of babies and small children know, this phrase has a literal meaning. Small children occasionally have explosive bowel movements which, literally, leaves poo everywhere. This phrase is also a family-friendly replacement to "sh*$ hit the fan," which presumably is only a figurative statement.
1. Johnny woke up in the middle of the night with a stomach bug. It was a real craptastrophe!
There was a craptastrophe at work. My project completely disappeared from my computer.