I've been thinking about this, and while I understand that I may be considered rude when I correct a grownup's grammar, I think it is much ruder to want to be uneducated or to complain about finding something out you didn't know.
Plus I try to do it by repeating what they said, only not wrong, as I go on with the conversation, so it's not as if I stop the conversation to say No, that is wrong.