One of Our Angels is Missing - S4-E16
Revealing mistake: Fake backgrounds and stuntmen are extremely evident throughout the entire episode.
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)