I'd like to think that the whole monsters in the closet/under the bed thing started with Crowley:
Crowley:*Under Aziraphale's bed probably grabbing something*
Aziraphale:*On the bed, stepping down*
Crowley:*Accidently grabs Aziraphale's ankle*
Aziraphale: AAAHH- SOMETHING GRABBED ME KILL IT KILL IT!
Crowley:*Jumps out from under the bed ready to battle to the death, then realizes that he was the one who grabbed his ankle*
Crowley:...
Aziraphale:...
Crowley: I'm gonna go fuck with some people