Revealing mistake: After Duke hears noise coming from the stairwell he locks the doors at the end of the hallway. After Duke shoots Parelli it cuts to the police firing at those locked doors, and when the doorknob blows off we can see that it's only a glued on doorknob, with no hole in the wood door, so it's impossible to have actually locked it.
Revealing mistake: When David is re-loading his gun whilst out duck shooting, we see three ducks flying to the right and then immediately to the left, revealing that it is the same film in reverse.
Revealing mistake: When Popeye's car stops chasing the EL train, there are skid marks from previous takes parallel to the ones that the car just made.