Factual error: A very common error in First World War films. British soldiers were not permitted to shave their upper lips until 1916 - in 1914 almost all (unless physically unable to grow them) would have had moustaches.
Factual error: The frozen treats used at certain points in this movie are labeled with nutrition labels found in the U.S. today. The labels looked extremely different in the 70's.
Factual error: In the movie, Marcus is out of high school, probably in his late teens, early 20's. Going by 50 cent's age, this would probably put the movie back in the mid 90's. When he flashes back, he's younger, put it in the late 80's. Well, in one sequence when he's young, he's playing with friends and has on red and white low Air Jordan 2's on. These were not made in this color until 2004. Same in another sequence when he's talking to Majestic while eating fast food, he's wearing some white and green Air Jordan 4's, which also didn't come out til 2004, which in fact is when the movie was filmed.
Factual error: Towards the beginning of the film, the Cashes receive a notice about their rent. It's the 1950's and the address has a ZIP code. ZIP codes were not introduced until 1963.
Factual error: Clyde is in the shop buying batteries for his microphones. The microphones we see later, are Shure studio microphones that do not use batteries. Batteries are very rarely used in studio microphones.
Factual error: In the scene where Brian gets fired from The Rolling Stones, Keith Richards sees a live album Jimi Hendrix and mentions it, Brian says Hendrix sent him one. But this scene must have taken place in 1969: Hendrix hadn't released any live albums earlier than 1970.