When you have an argument with someone but you are both arguing for the same side of the argument without realizing until afterwards.
e.g.
Person 1: duude Skyrim is like the best game.
Person 2: no way mate, Elder Scrolls is the best game!
*argument
continues*
Person 2:
wait a minute.. They're both
basically the same thing.
Person 1: ahhhh no way.. lol