Warner Brothers production notes for the film reported that three endings were shot. According to a November 1955 article in the Los Angeles Times, the end of the film was kept secret and the last five pages of the script were not distributed until ready to shoot.
The book Rhoda claims to have won in Sunday School, "Elsie Dinsmore," was a story with religious themes about a pious 8-year-old who, in sharp contrast to Rhoda, was obedient to her elders to an alarming point, even enduring verbal abuse from a nasty parent. It was written by Martha Finley in 1867.
Billy Wilder wanted to direct a film version based on the successful play, but couldn't get permission from the Production Code Administration. They objected because in his version her crimes went unpunished.
The trivia items below may give away important plot points.
The original ending had Rhoda surviving, and her mother dying. The Motion Picture Production Code in effect at the time, however, required that "Crime shall never be presented in such a way as to throw sympathy with the crime as against law and order." The usual interpretation of this was that criminals weren't allowed to "get away with it." Because of this, the ending was changed: Rhoda's mother survived being shot in the head, and Rhoda herself was killed by a bolt of lighting by a lake.
According to Patty McCormack, the "curtain call" of actors at the end of the movie was a holdover from the Broadway production. The original ending of the play, in which Christine dies and Rhoda lives to kill again, so angered Broadway audiences that when the performance was over, "they were almost literally ready to kill someone." The "curtain call," where Christine turns Rhoda over her knee and spanks her, was a way of breaking the tension and sending the audience off with a laugh, by having Rhoda get her comeuppance.