1. Number of the Beast :
After Alex is beaten by the homeless and his old droogs find him, they have numbers. Dim is Officer 665 and Pete is Officer 667. So, if Alex had taken a different path in life he would have been 666 !!
2. Space Odyssey Reference :
When Alex talks to the store employee about the album he ordered in the record store, the soundtrack to 2001: A Space Odyssey can be seen in the store background.
3. Author Reference :
Alex's name is mentioned as 'Alexander DeLarge', but during his release from prison newspaper articles call him 'Alex Burgess'. Anthony Burgess is the author of the book the film is based on.
4. Subliminal Messages :
When Alex kills the "filthy old soomka" with the big white phallus, the lady screams and you'll notice the screen flashes very quickly for a few seconds, then you'll briefly see an illustration of a mouth screaming inside of another mouth before the film continues. There are many similar strange, subliminal illustrations hidden within the movie frames.
5. Beethoven :
The doorbell at Alex’s house plays the first four notes of Beethoven’s Fifth Symphony.
6. Sound :
The first movie to make use of Dolby sound, it used Dolby noise reduction on all pre-mixes and masters, but a conventional optical sound track on release prints.
7. Cut :
Kubrick’s first cut ran almost 4 hours and all unused footage was destroyed per his request. Wrap : The final scene was done after 74 takes!!
8. Symmetry :
Barry Lyndon, Space Odyssey, Clockwork Orange .. the ending and beginning sequence of all these movies are interconnected visually!!!