Other mistake: As Frank and Betty move the wardrobe a chair tips over by itself. Frank and Betty are nowhere near it - it is presumably attached to a wire which is pulled by one of the crew, but has no significance to the plot.
Some Mothers do 'Ave 'Em (1973)
1 other mistake in season 1 - chronological order
Starring: Michael Crawford, Jane Hylton, Michele Dotrice, Jessica Forte
Genres: Comedy
Moving House - S3-E1
Revealing mistake: At the beginning when Frank is directing the vans, we see that one gets its windscreen broken by a ladder from another van. However, the glass breaks just before the ladder touches it.
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.