What to Wear When Yachting - S3-E7
Revealing mistake: At the beginning of the episode, when the TV repairman is at the door, you can see that the background behind him is a studio wall - not the front yard we normally see on that side of the house. This effect also appears in many other episodes.
Continuity mistake: When the milkman picks up the empty bottle, the note is flush with the top of the bottle, but when he stands up it it is sticking halfway out.