In an AP conference earlier this fall, one of the readers mentioned a peculiar difficulty some students had: They wrote too much. She said that on numerous exams, students who were trying to explain a phenomenon as directed by the exam would wander around (so to speak) and eventually stumble into the neighborhood of the correct answer. And then just when they were about to win the point, they would write something to obviously and irreparably wrong that the graders had no choice but to mark it incorrect.
Her advice for us to give to our students: Take a moment to think about the question, decide what the correct answer is, and write it down. Then stop.