A wrong answer can reveal an intelligent interpretation of an unclear question.

Wrong answers often contain good reasoning applied to a question that was not clear. The student read it differently, made a sensible inference, and followed it correctly, and the mark records only that the result did not match. What looks like a failure of understanding is sometimes a failure of the question.

When someone gets something wrong, ask how they got there before correcting. It takes a minute and it frequently reveals that the ambiguity was yours. And when you are wrong about something, explaining your route is more useful than accepting the correction.