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.
Some Mothers do 'Ave 'Em (1973)
1 mistake in The Baby Arrives - chronological order
Starring: Michael Crawford, Jane Hylton, Michele Dotrice, Jessica Forte
Genres: Comedy
Frank: I didn't know what a girl was until I was twelve years old.
Betty: You must of done.
Frank: How could I. I'd never seen one.
Trivia: Michael Crawford was the producers' third choice for the role of Frank Spencer. The part had been previously turned down by both Norman Wisdom and Ronnie Barker.