When Elaine takes the "window" dress back to the store, she is wearing white undergarments that are visible through the diamond shaped windows in the dress. She tries on another sleeveless dress and her brassiere straps are now black.
When Jerry enters the changing room to talk to Kramer, a wallet is visible secured in the elastic in the front of Kramer's boxer shorts. Moments later the wallet has disappeared.
When Elaine argues with the saleswoman and says, "That's preposterous!" a woman in the background sits down twice.
Kramer enters the changing booth to screen left. He peers at Elaine from this booth. Later it is revealed he has been in the booth for over an hour, but he is now in the booth screen right.
When Elaine walks out of the back room to show Jerry and George the dress, she takes about two steps and in the next shot is already standing right next to George.
It is supposed to be very cold when Elaine is outside wearing the dress. Snow is on the ground, but no one has visible breath.