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: The boat is quite rocky, causing their food to move around the table. However, the plate obviously has wheels on it to make it move easier. (00:18:00)