During the fight in the trailer, a rail on the kitchen counter gets broken. It is fixed a few minutes later.
When B-Rabbit goes to his mother's trailer for the first time, he is in street shoes. When he takes Lily into the trailer, he is in boots.
When Jimmy gets his sweater from behind the bin before the first battle, Cheddar Bob asks "are you going to stay at your Mom's?", then puts his cigar to his mouth. When the angle changes so you can see all of the 313, Bob is clearly holding his cigar down when it should be in his mouth.
In the basement, Future holds his cigarette between his forefinger and middle finger. In the next shot it's between his thumb and forefinger.
Length of B-Rabbit's sideburns.
When B-Rabbit and Alex are having sex in the factory, they separate for a few seconds and someone appears to run past in the background. That's actually Alex's arm moving.
When they are all in B-Rabbit's car and they start shooting paintballs at various things, the hopper, which holds the paintballs, is missing from the top of the paintball gun.
When B-Rabbit is using Sol's paintball gun, he fires it at the police cruiser's window, but the paint splatter comes from the opposite direction.
When Alex confronts B-Rabbit in the parking lot to chat, she tells him that she's heard that he's a real dope rapper but she already heard him rapping earlier in the movie just before B-Rabbit and his friends are confronted by the free world members. She later admits this to him. At first she just doesn't want him to know that she heard him.
The Chin Tiki restaurant and bar was closed in 1980, 15 years before the film portrayed it operating.
Set in 1995, but later model cars drive by in many scenes.
Detroit's casinos were built in 1999, 4 years after the movie's setting.
When Rabbit and his crew are driving through Detroit at night before they shoot the pintails, they pass a CVS Pharmacy. They were Arbor Drugs in 1995; CVS bought them out later.
Brittany Murphy's character's hairstyle wouldn't have been a thing in the mid 90's. It was a hairstyle that was popular in the early 00's.
In the first battle, the DJ plays the instrumental of "The Next Level (remix)" by Show + AG, which was released on PayDay records. The records shown on the decks aren't PayDay records. The DJ is using the same records in the end battle, when a different track is played.
When Jimmy Smith Jr. is talking outside the factory, the crew (holding the boom mic) is reflected in the window behind him.
A cable is visible in the last rap battle.
When Alex and Jimmy are having sex at the plant, a man is visible in the background briefly running around.
In the Rap Battle, Future calls Lotto and Lyckety-Splyt by their real-life rap aliases, "Ox" (Nashawn Breedlove) and "G L Strike Sanders", instead of their characters' names.