Revealing mistake: When Charlie and Grandpa Joe drink the fizzy lifting drink and are floating in the air you can see the hook that is connected to the wire that Charlie is being held up by.
Revealing mistake: When the bats invade Dr. Dunwoody's bedroom, you can see the strings on one of them.
Revealing mistake: When the platoon hear the church bells, they mistake it for the invasion signal and construct a barricade across the high street. If you look behind them you can see cars parked on the road in the distance that date from well after the 1940s.
Revealing mistake: A 'bald wig' is visible on Harry Morgan when he first arrives in Purgatory.
Revealing mistake: Just before Hermie goes into the drug store to buy the condoms, you can see his breath when he is talking even though it's supposed to be summer time.
Revealing mistake: When Stan drives through a puddle and splashes Blakey the water just reaches Blakey's knees but in the close up the water that goes in Blake's face is obviously thrown at him from a bucket.
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.