Other mistake: When Jerry, George and Elaine go out to Long Island to the party, George sees his lady co worker friend who says "guess what property I just sold...129 West 81st". That is Jerry's own address, which wasn't sold to anyone.
Continuity mistake: Jerry and George are at the pharmacy. As George pays the bill and is heading out, behind Jerry there's an old lady at the counter. The camera changes and the same lady disappears and you can see her in other side of the store heading again to the counter. (00:02:25)
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.