The Bermuda Triangle Crisis - S2-E4
Revealing mistake: When Wonder Woman drops a mine into the water, she's supposed to be dropping it into the ocean. However, you can see a drain plug of some sort in the background and the "ocean" is obviously a pool, or similar.
One of Our Angels is Missing - S4-E16
Revealing mistake: Fake backgrounds and stuntmen are extremely evident throughout the entire episode.
Revealing mistake: Our heroes arrive at an RAF base. Purdey hears a loud aircraft noise and looks up to the sky - cut to shot of a fighter jet. For a few frames after the cut, a caption that reads "The Engineer in the RAF" is just visible. It would seem they 'borrowed' some material from an RAF training film. (Indeed, a later shot of shells exploding was obviously taken from a videotaped TV programme, judging by the 'lag' effect caused by the tube camera that would've been used).
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)