Wendy House - S3-E2
Other mistake: When Frank, Betty and the insurance assessor are sitting on the sofa, it's obvious that all three actors are trying hard not to laugh when the assessor starts sinking into the sofa.
Some Mothers do 'Ave 'Em (1973)
1 other mistake in season 3 - 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.
Betty: Don't get so depressed Frank, you've only been out of work two days. Mr Rogers hasn't worked for six months.
Frank: Mr Rogers is 86 years old!
Trivia: Michael Crawford performed all of his own stunts in this series.