Revealing mistake: Near the end of the episode, all the castaways are passed out cold when Lord Beasley finally catches his butterfly. Right after saying the line "That's the trouble with you Americans - you simply don't know how to hold your tea" as he's exiting off camera, Mrs. Howell starts to open her eyes slightly, but then quickly shuts them again. Perhaps she thought the cameras stopped rolling.
Double, Double, Toil and Trouble - S4-E4
Revealing mistake: As Serena gets hit by the pie her real blonde hair shows underneath, also, she wears minimal make-up for the only time.
How to Succeed in Farming Without Really Trying - S3-E6
Revealing mistake: When the patroon is policing the area, raking and sweeping, they are not really raking or sweeping. They are just going through the motions but the rake and brooms are not touching the ground. (00:06:00 - 00:07:00)
Revealing mistake: After the car hits Herman, it falls apart. After the left front tire falls off, the prop holding up the frame is plainly visible.
Christmas with the Addams Family - S2-E15
Revealing mistake: When Morticia is playing the flute, the strings that operate the basket lid and rope are visible.