When Mr. Dunbar shoots at the fire extinguisher with a paintball, it is a bigger cylinder than the one shown at the rest of the scene. And this smaller cylinder also doesn't have any paint marks on it.
When Mr. Dunbar is hiding behind the soda machine, the mirror he is using to stealthily look at the corridor is completely at the wrong angle for it. He is pointing the mirror totally at himself.
During the parking lot conversation scene between John and Kim, he says that she left the bicycle when she went to pick up her last boxes, implying that she lived at the house he is currently living. But right after that he asks if her new boyfriend is using his park spot at the house that she lives now, implying the opposite, that he was the one who left their residence.
Mr. Dunbar set the paintball game rules saying that head and chest shots are forbidden, only arms and legs are allowed. During the game no shot other that directly to the chests are fired, including Mr. Dunbar ones. And when the game ends no one complains about the broken rules. It seems that the film crew just ignored the rules established on the dialogues.