Revealing mistake: When Laura picks up Baby Rose from Thomas Stark, the blanket is flipped up and you can see a plastic doll's leg, then when she goes into the bedroom, she has the real baby.
The Werewolf - S1-E5
Revealing mistake: It is obviously a dummy in a woman's dress The Werewolf throws into the empty swimming pool.
Revealing mistake: When Arnold's burns down you can see the set walls wobble a lot.
Revealing mistake: The actor that played Barney Miller, Hal Linden, was left handed and did everything left handed. However, they always put his gun on his right hip with the handle facing the right. A lefty cannot conveniently pull the gun this way.
Suggested correction: Hal Linden writes with his left hand, however he uses his right for many things. He ate with his right hand (in Graft), and in the episode where Fish thinks he needs to retire, Barney pulls his gun with his right hand.
Revealing mistake: It's obvious by his hand movements that Gunner Graham isn't actually playing the piano.
Suggested correction: If I take some random episodes, S7E5 (01:54) or S3E1 (21:51) or S7E1 (13:21), it's either real or at the very least the touch matches the notes in time. This is all on YouTube for easy check-up. S8E2, which centers on him playing something complicated, doesn't show his hands, so avoids a mistake. Glimpses during opening credits are less well-matched over the seasons, but then, that is supposed to be the score only. Claims that it's all very obviously fake just don't seem to ring true.
Suggested correction: I have recently watched the whole run of the series, every single episode, and I do not agree for two reasons. First, there are no close-up shots of hands on the keyboard. Sometimes it's bound to be a recording, for example when the monsoon breaks and the piano is full of water. But that is not a mistake. Second, the show released its own LP record, and actor John Clegg is credited for playing one musical number on that album. So I'm not so sure if he wasn't the real pianist all along. It makes sense that they'd cast someone who could actually play well enough.
If you look carefully, it obvious to any one who plays the keyboard, he's not playing. Also its as plain as the nose on his face that in shots with him playing the accordion, it's a fake.
Of course the accordion is fake - that is just the one episode where they were sent to the front line, so they needed to do something. But I'd very much like to get an episode & time where it's "plain as the nose on his face" please, apart from special situations like in the pouring rain. (I repeat, no-one else is ever credited for the piano, not even on the record album; and it would be strange casting then, since he was always the mediocre pianist from day 1).
The Dark and Bloody Ground - S1-E3
Revealing mistake: Rockford puts the newspaper snippet in front of her client. The poorly constructed prop has the first 2 paragraphs a perfect repeat of each other. (00:28:25)
Revealing mistake: The bare, non-furry, forearms of the actor playing Anto are exposed when he is entertaining the gorilla soldiers.
Revealing mistake: When Major Popov aims the rocket launcher at the Venus probe, there is no rocket in the launcher- it is just an empty tube.