<p>I agree with marian regarding honor codes. My institution’s honor code would suggest that reporting an error is required; otherwise, the student is getting an unfair advantage. I once had a student report a large error in his favor. When the TA was toting up the points on an exam he gave the student 20 points extra. I was impressed that the student reported it to me. After deciding how to deal with the case, I polled several colleagues to ask what they would have done. We all went for the same compromise: give the student half of the points.</p>