The Contest - S4-E11
Revealing mistake: In the scene where Jerry and Marla are making out on his couch, at the top right window you briefly notice someone walking by. This is impossible because The Apartment that Jerry lives in is on the fifth floor.
Answer: The name is Delores. He shouts that name at the end of the episode. Then in a later episode, "The Foundation", he runs into her. And he repeatedly calls her Delores.