Final appearance of Frank Flannigan.
In the scenes taking place at a TV station, there are television cameras in the studio with the call letters "WPIQ-TV" painted on them.
This is an "in-joke", since the call letters mean "Private Investigator Queen".
Ironically, in real life, there has been a New York television station called WPIX-TV since 1948.
A double trivia/goof. When Ellery goes to the blackboard to draw a diagram, the drawing is done in less than 5 seconds. Even though it is a very detailed diagram.
Every so often Flannigan tells Vera, "You're a good secretary."