Character mistake: During Mickey's MI it's stated that his blood pressure is 200/50 (that alone is ridiculous) and the doctor immediately says "OK. Let's start CPR." If Mickey has a BP he has a pulse, and any CPR certified healthcare provider would know that you can't do CPR on a patient with a pulse.
Revealing mistake: During the movie's opening title sequence, Clubber Lang knocks out Jaffe in the sixth round. If you watch carefully, you can see that Lang's boxing glove actually hits Jaffe's shoulder, but he still knocks him out in the process.
Revealing mistake: In a shot of Clubber Lang and his manager standing in the corner at the end of the first round during the rematch between Lang and Rocky, "Clubber" on Lang's boxing shorts and "Heavyweight Champ" on the manager's shirt are back to front, indicating that it is a reversed shot.