The whole "posing in contradiction with your skills" thing is not unique to social conflict. You get it all the time in any sort of skill-based roleplay, whether it's someone trying to fake playing a doctor or a soldier. It works both ways, too -- someone with RL knowledge letting that bleed over into their character, or someone without any knowledge trying to fumble their way through and making a mess of it. I think it's just more glaring in social situations because a) social situations make up, what, 90% of MU scenes? and b) we are all skilled to some degree in socialization, making our BS-meters more sensitive.
To tie back to what @Thenomain said, this is where the lack of a central authority or social contract bites games. You don't see it as much in ST-ed scenes because the ST has the authority to be all: "Yeah, no, there's no way that argument is going to convince the guard.." And you don't see it on consent games as much because the social contract says "If you want to convince my character of something, you need to get me, the player, on board with why that's a cool story."