Revealing mistake: After the piano falls from the ceiling, Ritchie rumbles Eddie's "plan" and kicks him out of the flat. When Eddie is pushed out the front door and down the stairs you can see him falling to begin with, but halfway through he actually stops as if he's kneeling, then ducks out of shot. The sound effects lead us to believe that he's fallen all the way down the stairs and been attacked by a pack of dogs at the bottom.
Revealing mistake: During a few of the "flaming fart" scenes, you can see a wire connected to the bottom of the actors. It's hard to see because it appears that it was attached to their legs in some way, but you can still see it when they move around a lot. The easiest place to see it is when the "devil" knocks on the door and they all fart and jump around, look carefully between/to the side of eddies legs for a black wire. It's not the dressing gown belt because that's tied around their waists.