Cold Comfort - S2-E2
Revealing mistake: In the very beginning of the episode you can see Mr. Grainger sitting in a chair to the very far left, with his head down, waiting for his cue to walk-on.
Cold Comfort - S2-E2
Other mistake: At the very beginning you can see a girl in the background with long hair, pausing to speak and gesture to someone off stage, before she removes the sheet draped over the counter.