After Leon rescues Mathilda from the SWAT team and dodges the snipers, he shoots out a ventilation fan to make an escape route. He gets off two shots before we see the vent, but when it does we see about 7 bullet holes around the vent. Then the camera goes back to Leon as he continues firing. When the camera switches back, the holes around the vent are different.
There are many similarities between A Clockwork Orange and the film Leon. Both main characters 'Alex' and 'Leon' are seen drinking milk many times in the films. Stansfeild (Leon) and Alex (ACO) both listen to Beethoven to stimulate their violence and the song 'Singing in the Rain' is featured in both films.