While inspecting the contents of the victim's suitcase, Miss Marple finds flyers for a theatrical production of Agatha Christie's "Murder She Said": this is the title of the first movie in which Rutherford appeared as Miss Marple and is an in-joke as - despite Jim Stringer's claim that it's "an excellent play" - Christie wrote no such work. Some foreign-language prints give the play's title as "4.50 From Paddington", though this was similarly was never adapted for the stage.
This was the penultimate production in the series of four films with Margaret Rutherford as Miss Jane Marple. The last is Murder Ahoy (1964) (made the same year as Murder Most Foul (1964)), in which Inspector Craddock has been promoted to the rank Chief Inspector. After the series concluded Rutherford and her husband Stringer Davis reprised their roles of Miss Marple and Mr Stringer only once more, for a brief cameo appearance in The Alphabet Murders (1965).
Miss Marple's audition piece for the Cosgood Players is her dramatic rendering of "The Shooting of Dan McGrew", a 1907 poem by Robert W. Service. Rutherford was especially fond of the piece, and reportedly once intended to give a reading of it at a women's prison to cheer up the inmates.
The music playing at the opening of the hospital scene is a reference to the television show Dr. Kildare (1961), down to the shot of the doors to the ward. The score alludes to the theme music from the series, "Three Stars Will Shine Tonight", composed by Jerry Goldsmith.
Cosgood hopes for a play-run longer than "The Mousetrap" - Agatha Christie's famous stage-play, which has been in continuous performance since 1952, the same year that source novel "Mrs McGinty's Dead" was first published.
There are various references to the work of William Shakespeare: the title hails from "Hamlet" Act I, Scene V (lines 27-28) where the Ghost speaks: "Murder most foul, as in the best it is;/But this most foul, strange, and unnatural." A key phrase in the film hails from "Romeo and Juliet" ("What's in a name? That which we call a rose/By any other name would smell as sweet" (Act II, Scene II)). And Cosgood alludes to "Macbeth"'s line "Sleep that knits up the ravelled sleave of care" (Act II, Scene II, lines 34-35).
According to the closing freeze-frame, the Milchester Players are to perform "Murder Without End". Consequently some have interpreted this as the title of a further, unmade, entry in the Miss Marple series - possibly based on Agatha Christie's 1942 novel "The Body in the Library".