Arse over tit
British SlangDefinition
Vulgar British slang meaning to fall headfirst in a tumble; by extension, for a situation to go completely wrong.
Examples
"He slipped on the icy pavement and went arse over tit into a snowdrift."
"The product launch went arse over tit when the website crashed five minutes in."
"I leaned back too far on the bar stool and ended up arse over tit in front of everyone."