When Alex and his droogs come in on Billy Boy making ready to rape the young girl, the first shot shows the lacing of a shoe or sandal on the girl's left calf, but afterward it jumps to her right.
According to actor Malcolm McDowell, director Stanley Kubrick was concerned that the Droog attack on F. Alexander and his wife would become just another dark, cruel and violent scene in a movie that was already full of dark, cruel and violent scenes. Kubrick wanted the Alexander home invasion to stand out as genuinely horrifying, but he was at a creative impasse. After Kubrick shut down production for several days to ponder the problem, he thought of Alex dancing during the attack. Malcolm McDowell suggested dancing (and singing) along to "Singin' in the Rain," as it was the only song to which he knew all the words.