This ^^^^^. As my grandfather used to say, correcting another person's pronunciation or grammar in public is very poor form indeed.

To give an example, I could point out the comma splice in MikeyBoyAZ's sentence "The pronoun whom is used as an indirect object, this is easiest to remember with the addition "to or for" [whom]." I wouldn't think of doing so, but for the fact that he who committed this elementary error had taken it upon himself to correct a relatively trivial error in another poster's message.