Eddie Dupris claims that a knockout is caused when the neck turns a certain degree after which the human body takes over and shuts itself down for safety reasons. While generally unknown, it is widely believed that a knockout is actually caused by the rattling of the brain against the skull causing severe trauma resulting in a concussion.
When Eddie is reading a magazine, while on his bed, Frankie enters the Gym with a hamburger. Eddie throws the magazine on the bed to see who has entered. The magazine is seen on the pillow, wide open. The magazine is next seen closed and almost out of sight.
The first scene with Father Horvak is an overhead shot of him leaving the church talking with a parishioner. The parishioner has a bag in her right hand. The next shot switches to a head-on shot of them a second later but the parishioner has the bag in her left hand, too fast for her to have switched hands between the two shots.
When Maggie is taking her mother to the new home she just bought her, there are palm trees around the other houses in the neighborhood. This scene is supposed to be in a small town in Missouri, where palm trees do not grow.
When Frankie & Maggie leave Maggie's hometown of Theodosia, MO, they pass a road sign with "160" inside a Missouri outline, the correct sign for Missouri State Highway 160. However, the main highway through real-life Theodosia (in far southern Missouri, just north of the Arkansas border and east of Branson) is U.S. Highway 160.
When Frankie is training Willy in the ring, Maggie walks in to begin training. She is carrying a new, expensive Ogio gym bag even though she's supposed to be so poor she has to scrape change together to buy a speed bag.
The goof items below may give away important plot points.
When Frankie removes Maggie's ventilator to administer a lethal dose of adrenaline, she is clearly breathing on her own. Earlier in the movie, Eddie's voice-over noted that Maggie will never be able to breathe on her own again.