The Werewolf - S1-E5
Revealing mistake: It is obviously a dummy in a woman's dress The Werewolf throws into the empty swimming pool.
The Countess - S1-E4
Revealing mistake: In the climax of the episode, the car goes off a cliff when Rockford steers abruptly. Definitely not your ordinary maneuver, but other people must have chosen to commit suicide in that spot, since there are previous tire marks of a sudden turn in that exact spot. (00:49:20)
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.