When Meg is walking down the staircase to the janitor's apartment, she is wearing open-toed mules. In the scene in the apartment itself, however, she is wearing regular pumps. (Fact: the feet in the shot on the stairwell are actually those of Doris Wishman; they can also be seen in the Port Authority crowd scene).
Director Doris Wishman and the camera operator can be clearly seen in the large glass window when Della tries to dance with Meg.