During the most of the movie, Randy wears a blouse is buttoned up to the neck and a necklace over the top of it. Except for the scene where she is exiting the casino after her big loss, as she walks out and her blouse is unbuttoned at the neck and she is not wearing the necklace. However, in the next scene the blouse is buttoned again and necklace returns.
The description of an apparent discontinuity is accurate; however, in the shot with open blouse and sans necklace, Randy is also not wearing her jacket. In the following shot, as she emerges from the casino with Jack after her devastating setback, she has donned her jacket, buttoned her blouse, and restored her necklace. The apparent costume discontinuity dissolves in the brief lapse of unrecorded time.