What We Do - S15-E18
Revealing mistake: Sam's mother is standing at an open window, smoking a cigarette. The outside of the window is covered with some sort of frosted glass or ice or something (making it pointless to have it open anyway). Two shots later, we see her close the window from a different angle. This time, the window is clear and you can see the brick building next door. (00:19:36)
Answer: It was in "Chaos Theory", the first episode of season 9 (time code 00:13:55).
Super Grover ★