A wannabe Grammar Nazi who uses incorrect grammar but fails to see the error in his/her own typing, yet always criticizes others' grammar errors. They can be characterized by the idiom: Pot calling the kettle black.
Me: "Sometimes I like to wind grammer natzis up just for the hell of it. Especially when I call them a grammer natzi. That works well."
Friend: "What a good cant".
Someone who takes every opportunity to correct someone on their grammar, even though they themselves don't know how to spell "grammar." See also hypocrite.
Impostor: Hey guys, don't you know the difference between your and you're? I'm such a grammer nazi.
Sane guy: Wow, you have no idea how stupid you sound.