When Barry walks away from Joy after he first fakes heart problems at the outside market, Joy's voice can be heard telling the salesperson in French that she wants celery. But as she is turning her head to watch Barry leave, you can clearly see that her mouth is not moving.
Mitzi Gaynor breaks her picture over Gene Kelly's head, and storms out the door. As he gets up to go after her, the frame is still clearly around his neck. But as he goes out the door, the frame is gone.