When the studio of the missing cartoonist is shown, careful viewers will see various props (paintings) used in other series entries: The Case of the Crying Cherub (1960), The Case of the Purple Woman (1958), and The Case of the Reluctant Model (1963).
In the opening scene, as Victor Buono and Carl Don as two beatniks are talking, Buono takes a drag off of a cigarette and passes it to Don, who takes a drag then leisurely blows the smoke out - a subtle, but apparent indication that they are smoking marijuana.