Revealing mistake: When Henry VIII runs out of the cowshed from the irate farmer, you can see it is not Sid James but his stunt double.
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.