Revealing mistake: Season 3, Episode 68, "The Duel": The prop wash from the helicopter filming the overhead scenes are evident as everything (dust, twigs, leaves, etc.) is being blown around.
Revealing mistake: When Albert and Harold are fighting over the cracker Albert's face goes into the trifle, and if you look carefully we can see that he just lets go of the cracker and puts his head in the trifle deliberately.
Water's Edge - S3-E3
Revealing mistake: The murder victim's skeleton, which Rusty finds in the attic, has a clear cut all the way around its skull. It also has metal pins holding together what turns out to be rubber bones: one of the ribs wobbles up and down when Rusty brushes against it with the cash box. (00:42:00)
Revealing mistake: Lucy and Vivian are trying to retrieve a $500 bill an elephant is standing on. They need to get the elephant to lift its leg. "Vivian says that won't work, that won't work." You can see the elephant has lifted its leg and the $500 bill has stuck to its foot.