While Mattei is in the hospital the retrospect shows a young boy wrapping the gun handle with electrical tape. The idea is to make it impossible for the police to pick up fingerprints after the killing. However, he's using the wrong kind of tape - the smooth one instead of the sticky rough one - which renders the whole process useless.
Towards the end of the film, a man is eating a slice of pizza and telling a young man playing a video game what to do. He turns around and the slice he is eating is suddenly less eaten than it was before.
When the gang shoot Reno's character with the 22 bullets, they also use a shotgun. When the bullets are removed, they are all bullets. Shotguns don't fire bullets, they fire pellets. Plus, all the bullets removed are not deformed as they would be if they were actually fired.