From Alexander the Grammarian, I learned not to be harsh with people who make language mistakes. Don't call out someone for bad grammar or wrong pronunciation. Instead, find a smooth way to show the right form. Answer their question using the correct word, or confirm what they said but with proper speech. Don't make a big deal of their mistake. Just give them a gentle, indirect hint about the right way to say it.
From Alexander the Grammarian, to be un-reprovable myself, and not reproachfully to reprehend any man for a barbarism, or a solecism, or any false pronunciation, but dextrously by way of answer, or testimony, or confirmation of the same matter (taking no notice of the word) to utter it as it should have been spoken; or by some other such close and indirect admonition, handsomely and civilly to tell him of it.