This is the official term to describe that moment when someone walks near, into, overhears your conversation at the worst possible moment, even if the subject is benign.
Person A: So I was watching this movie yesterday, and there was this hooker snorting a line of cocaine of the table.
Person C: Whoa! Hookers and Cocaine? What did I just miss?
Person B: Nothing, dude. Just proving the Couch Theory.