One of Our Angels is Missing - S4-E16
Revealing mistake: Fake backgrounds and stuntmen are extremely evident throughout the entire episode.
Revealing mistake: Jamie prepares to split a giant heavy log in two, with her bionic arm, so that it will fit into her fireplace. Just as her arm comes down, you can see the middle seam where the log was pre-cut for the shot, so that it easily breaks in two. (00:00:30)
Revealing mistake: The set where Augustus visits Agrippa on Lesbos for reconciliation is the same set where Augustus already was before - he was there earlier, reading the news of Marcellus' death while he was away from Rome to visit the Eastern provinces. But as stated, he had not visited Lesbos at all, because he and Agrippa had temporarily fallen out. (00:39:30 - 00:41:30)
Revealing mistake: In the arcade the killer talks to his helper while playing a pinball machine. He plays very agitated and strongly. But if you look closer at the below end of the playfield you see that there is no ball and the flippers aren't moving. (00:23:50 - 00:24:48)