In Mrs Montenegro's first meeting with Lou in his office, she keeps changing position between shots.
When Lou and Marlene are talking in his apartment, her scarf keeps changing position from surrounding her face to trailing down the back of her head.
The calendar in Lou's apartment, from Tom and Jerry's Diner says 1940, however the dates match with 1941.
The movie takes place in 1940, yet the song sung in the nightclub is 'La Vie En Rose' which was not written until 1946.
Approximately 45 minutes in, Lou meets with the Madeline Kahn character in his office. A male member of the crew is visible, seated on a couch. He is in a pink shirt, wearing eyeglasses and appears to be reading a script. He is only visible for about a second, as the camera pans across the room. From contemporary photos it looks very much like Neil Simon himself.