Antoine Doinel (Jean-Pierre Léaud) makes a phone call at a pay phone and asks to speak to someone named Eustache. When Eustache comes to the phone, Antoine calls him Jean. Jean Eustache would direct Jean-Pierre Leaud in The Mother and the Whore the next year.
One of the composer portraits decorating the Doinels' front room is actually a portrait of the actor Oskar Werner dressed as Mozart for a play. Truffaut directed Werner in two films, Jules and Jim and Fahrenheit 451.