Micheal Jackson did do a Pepsi commercial where his hair was on fire caused by an electrical fault, but he did not fall down any stairs. He had to take prescription medicine to manage his pain.
At the pool party at Diana Ross' house, Marlon and Michael are talking to each other in the pool but their mouths don't move. When Joseph calls them out of the pool, Michael's mouth doesn't move when he says "5 more minutes".
During their performance at the Apollo, the strap on Jermaine Jackson's bass snaps off, nearly dropping the bass guitar. But since Michael Jackson's character is obviously the focus, this scene was never corrected.