In the comic book Dylan drives a white convertible Volkswagen Beetle with a black roof. In the movie it is a black convertible Volkswagen Beetle with a white roof.
In a photograph in Dylan's flat, we can see him and his assistant masked as Groucho Marx. In another scene, we can see that Dylan keeps his weapons stashed in a safe behind a Duck Soup (1933) poster (a Marx brothers movie). Both are references to the Dylan Dog comics on which this movie is loosely based on. Although in the comics, Dylan's assistant, sidekick and comical relief is not the zombie from the movie, but a stage actor who looks exactly like Groucho Marx, calls himself Groucho, and always dresses and behaves like the legendary comedian (it's implied this is due to the character's slight insanity).
When Dylan takes the handgun from the disgruntled husband early in the film, he is shown making the weapon safe. However, by racking the slide BEFORE ejecting the magazine, this would eject a cartridge but immediately replace it with another. So unless the businessman came with only one bullet, the gun would still have a bullet left in the chamber.
Dylan Dog: See? That's just what this case needed. A seven-foot tall, flesh-eating zombie. Which begs the question, are there any actual people left in New Orleans?