Deliberate mistake: While Donnie is training at Front Street Gym, when Rocky repeats, "One step at a time, one punch at a time, one round at a time," the closeup of two children watching from outside the ring is a flipped shot, note the backwards Nike swoosh logo and Everlast ring rope spacer. (00:53:05)
Factual error: When Creed and his girl go to the sandwich shop the cashier tells him it's $10.95. He hands her a $20 and she gives him back a $10 bill. Not only that, there is no way you would get two fully loaded Philly sandwiches and drinks for $10.95 in Philly.
Character mistake: In the final fight at Goodison Park, the commentator says 100,000 people are in attendance. Capacity of Goodison Park is 40,000, and even allowing for people in the "pitch" area around the ring there's nowhere near space for that many, unless there was another layer of fans standing on people's shoulders.