Incorrectly regarded as a goof that Frank shouldn't know Dr. Morgenstern, but Frank appeared in season 1 episode 1 as a police officer still when Dr. Morgenstern was still at County General and before Frank retired from the police force and became the clerk at the hospital. It isn't out side of the realm of possibility that Frank would have encountered Dr. Morganstern multiple times in the years preceding the show "timeline" of the show.