It's Comedy or Cabbage - S3-E5
Continuity mistake: When Midge descends the hotel's grand staircase, the piano player and guests are below. Once down the stairs, Midge finds Abe and Rose sitting in chairs with their luggage, just to the right of the piano, in front of a large pillar, with a round sofa behind. However, when Midge was walking down the stairs, the lobby looks different. The piano is in the same position relative to the stairs, but her parents and the chairs are missing and the round sofa is a different style and the shape of the carpeting in front of the pillar is also different.
