"Nobody cares!"
God I fucking hate this one. Someone starts an argument with you, so you start to actually explain your point of view to them, or disproving theirs, using facts, evidence, and logic in the hope that you might achieve something more than an endless back and forth of "You're wrong!"/"No, you're wrong!". But then, when it twigs that they don't actually have any ammunition to back up their opinion, they bail with something along the lines of "Don't shit yourself about it. Man, why do you have to get so serious about everything? Nobody cares!"; and then they act all smug and superior, as if quitting before you can lose is equivalent to a victory. Best part is, if you refuse to accept this bullshit tactic and continue regardless, you now get painted as an uptight wanker, and they just get to sit there, marinating in their own ego in front of their equally idiotic friends.
Oh, how I would just love to be able to reply without consequence... "Well, obviously you care, otherwise why would you start the fucking argument in the first place, let alone bail because you're too afraid of looking bad in public as your precious little opinions get torn to pieces by the guy who's actually thought his side through, YOU FUCK?!"