One More for the Road - S3-E22
Continuity mistake: In this episode you see Carol and Doug discussing Charlie over phoning and chart-reviewing. Carol is standing beside Doug. Suddenly somebody calls from behind the desk. It's Tommy, a "friend" of Charlie. When he's in the picture, you see Carol sitting in a chair, instead of standing. If she had moved to the chair, she must have done that SO fast, it's impossible.
Answer: It was in "Chaos Theory", the first episode of season 9 (time code 00:13:55).
Super Grover ★