Occurs when a person, usually of private schooling (Harvard/Yale/Princeton/etc), in an effort to show their superiority corrects a person's usage of grammar/verbage/word choice in spoken conversation. The reason for this is threefold: A) Their education costs a ridiculous amount, and while this education is in no way superior to a public school education, they feel the need to try and show how amazing going to a private school was by becoming the grammar police. B) This is usually done to hide their huge inferiority complex because they spent a shit ton on their education and that still does not make them special, and by correcting others in such mundane topics harkens back to days of yore where they sat at the head of the class as the teacher's pet. C) No one has been paying any attention to them for the past 5 minutes and their fragile narcissistic personality can not handle the 5 minute descent in obscurity by not being the center of attention.
"Me and Dan are headed out."
"Excuse me but you should say 'Dan and I are headed out' that is proper English!"
You have just been Harvard Mouthed/That person has a bad case of Harvard Mouth