A construction worker is putting up a plaque on the wall that says “do not post signs on this wall”, but is interrupted and asks a bystander to hold up the sign in place against the wall while he goes off to do something. A cop comes by, and tells the bystander that they can’t put that here because of the sign. When the bystander puts it down, someone else comes along and puts a sign up and the cop is fine with that because there’s nothing saying you can’t put signs up.

It’s basically an experiment on the psychology of bureaucracy.