When you realize part-way through an argument that you are actually both arguing the same point.
person a: No, there are 72 original episodes of Star Trek!
person b: No, because the pilot was a two parter!
person a: Yeah, and two parters count as one episode!
person b: But the pilot didn't get spit up into two parts until later, so that means that there were originally only 72!
person a: No! .......Wait, that's what I said.
person b: Huh?
person a: We're agree fighting again.