Wow, so many opinions. I'm not surprised as we are all so different. For me, it would depend on what the mistake was. Cut off points and so forth...no, I don't want to know because I already know and no one is berating myself worse than myself (ok, that sounded a little Dr. Seuss to me but anyway....) for blocks turned the wrong way, yes, a mention would be desperately wanted. Outside of that try to gauge the individual. Do they seem the sensitive kind? Then just be kind in your wording letting them know you really are intending to be helpful. Maybe even start out with a complement.
