Everything Old Is New Again - S1-E26
Continuity mistake: When people start to dance at the reception, there is a shot of Susan, Doug and Mark. Then the sax player approaches Susan and asks her to dance so she leaves and hits the dance floor with him. They show them dancing, then a few seconds later go back to the people standing by the table and Susan is next to Doug again. (00:42:00)
Answer: It was in "Chaos Theory", the first episode of season 9 (time code 00:13:55).
Super Grover ★