While being questioned in the pub you can see a television high on the back wall of the bar. The newscaster is just about to describe what Bunny Lake was wearing before she was reported missing when the channel is suddenly switched to a young British group (The Zombies) playing a very catchy song with lyrics that actually relate to the plot of the movie. Much later in the storyline Ann Lake escapes her hospital room through a basement level maintenance room where she runs past an old janitor sitting at a workbench listening to a transistor radio and the same exact catchy song is being broadcast again.
It was Keir Dullea's performance in this movie that led to Stanley Kubrick choosing him to play his most famous role of Dave Bowman in 2001: A Space Odyssey. He didn't even have to audition; Kubrick simply asked him if he wanted the role and he said yes.
Based on a 1957 novel by Evelyn Piper. While the book and movie share the same story hook (a little girl is missing and authorities doubt she ever existed), they have different settings (the book is set in New York, the movie in London), different characters, and a different solution to the mystery.