Revealing mistake: When the second piece of scenery falls on Frank, it has a tear halfway down and a hole in it that Frank's head goes through. A few seconds earlier, these weren't there. Maybe they were put there to make sure Frank's head went through, or else this is the second take.
Revealing mistake: While Frank is taking his driving test, he and Mr. Hayes are wearing standard road safety belts in the car. But when the car is in the sea after crashing through the fence, both the occupants are wearing motor racing type safety harness obviously fitted for the stunt.
Revealing mistake: When Betty goes into labour in the night and goes downstairs with her suitcase, she tries opening the front door. You can see it moving with her pulling. And then when Frank goes to climb out of the window to follow the key, he climbs onto the sink, which almost rolls away, which reveals the fakery of the set.