The Library - S3-E5
Continuity mistake: When Bookman is lecturing Jerry in his apartment, he picks up a navy book with gold lettering and places it on the bookshelf in front of a pink book. For the rest of the shots, the navy book disappears and the pink book is back in front.
The Library - S3-E5
Character mistake: When Marion mentions Mr Bookman she says he'd been on the job for 25 years. Yet later on he tells Jerry he's been on the job since 1971, which in 1991 makes it 20 years.
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.