Plot hole: Early in the movie, the principal from Precious' school buzzes Mary's apartment intercom to talk to Precious about alternative education. In order to hear what the teacher is saying Precious has to press the 'Listen' button, but occasionally the teacher can be heard speaking through the intercom even when Precious isn't holding the listen button.
Other mistake: After Precious has her baby, Abdul, her school friends visit her in the hospital as her nurse, "John," is eating lunch with her. When John introduces himself to Precious' friends, he says he is a nurse's aide, and then later refers to himself as "Nurse John." When he stands up to leave the hospital room, you can see the word "NURSE" on his hospital ID badge pinned to his scrubs.
Character mistake: While Precious is in the hospital, her teacher is writing a response to Precious' journal entry in which Precious had written about her grandmother having referred to her as a "dog." In the note the teacher wrote, it says "Dear Precious, You are not a dog. You are a wonderful young women." instead of "You are a wonderful young WOMAN."