It's bad manners to comment, not to find it annoying, I think, helly.
I think duck has a point, but I also think it's simply unnecessary to make people feel bad. I know there are a few rare people who'll respond with 'oh, thank you, I'm so glad you corrected me', but honestly, far more will feel hurt or upset.
It is also hypocritical unless you never make a mistake yourself, and I have to admit, every time these threads come along, I am stunned by the number of people who're quite willing to criticize other people, but who get incredibly defensive when it's pointed out that they, too, make mistakes. I think it is fairly telling, that! It suggests people do know it's upsetting (because they get upset themselves) - they just assume they'll not be on the receiving end.
tufty, that's so sad, but wonderful you were able to do that with her. You must feel very proud.