Not just "fucking up really bad." Used to emphasize the way that the really bad fuck-up harms or humiliates the person who fucked up. The idea of shitting the bed is not just a) that the person intended to do one thing but instead laid a giant turd but also b) that the person laid that turd in their own bed and now has to lie in their bed of shit.
Darlene merely laid a turd by flunking the algebra test. Mike completely shit the bed when he cheated on the test and got caught and expelled from school.
One night after drinking heavily, I decided to give myself 2 IV bags cause I was super drunk. The good news was that I woke up without a hanger, bad news is that Purdued the bed. They really Shit the Bed
An odd and peculiar sexual fetish where the woman is on her back and rolls on to her shoulders presenting her pussy to the sky. The man, or woman then steps between her legs into a scissoring position and then proceeds to fire a hot log right into her vagina. This obviously only works if the turd is relatively hard and/or the pussy is extremely loose.