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.
Wendy House - S3-E2
Revealing mistake: When Frank is talking to the postman at his front door, it's so obvious it's a studio set. The background seen outside the front door is painted scenery.