Continuity mistake: In Maggie's first fight (when Frankie isn't her manager), she comes off after the first round and takes her mouthpiece out. After talking to the manager, she gets back up to enter the ring without putting it back in, but when the bell rings she has it in again.
Continuity mistake: When Maggie is talking to Frankie in the gym bleachers, discussing Frankie's daughter, Maggie's ponytail keeps shifting from behind her back, to over her shoulder, and back again.
Audio problem: During any fight when people are yelling "Mo cuishle", watch the spectators around and you'll notice that no one is mouthing the words, or is in sync with those words. Even if the yelling came from offscreen (i.e. from the farthest bleachers) the noise should have sounded from far away, not as neat and close as it sounds.
Revealing mistake: In some of the scenes where we see Maggie in the hospital bed, you can see that her breathing is not totally in sync with the breathing machine (i.e. her chest goes up before you actually hear the machine blowing air into her).