This... this is not a post to which I should be responding. It is with great willpower that I present but one, gaming-specific answer:
Players speaking in-character using modern vernacular - the type of drivel they'd put in a 'tweet' or a 'text', spewing forth from their characters' mouths.
Luckily, I only experience this when running games for new groups down at the FLGS, and not at my home. I try, with great patience, to explain to them the goals behind immersing yourself into the mind of the character, affected appropriately by his or her surroundings. I certainly don't mean to sound mean, or rude with this particular pet peeve - I understand they are new players, so I treat it with dignity and the kindest of educational process I can offer. That does not stop me from wiping away the tears of pain when I get home after the game.
That is all, until my resolve falters and I'm forced to rant in ways I wish to not.