The error lies in the use of a double negative. The word "forbade" (the past tense of forbid) is inherently negative, meaning "to command someone not to do something." In English grammar, when a negative verb like forbid, prohibit, or prevent is used, it should not be followed by the word "not" in the subordinate clause, as the two negatives would technically cancel each other out or create a redundant, confusing structure. To correct the sentence, it should simply read: "She forbade him to enter that room." This correctly conveys that he was ordered to stay out. Using "not" after "forbade" is a common trap in competitive exams designed to test your understanding of redundant expressions.