People who think their dialect or language style is grammatically correct and others are wrong, because they don't personally known the grammar rules of any other dialect or language. They don't understand that language is alive and evolving and that the purpose of language is communication.
I would add to this people who aren't able to use their own native language properly. I receive IMs at work all the time that sound like a 16-year-old is sending a text message. Maybe I'm just getting old, but half the time I honestly can't decipher what they're asking me.