As Dawson and Joey talk, and the camera switches between them, her arm position changes. When it faces her, her arm are crossed in front of her. When it faces him, her arms are crossed behind her. Back and forth, back and forth.
Actually, The arm crossing/changing could have been used for effect.. Showing her arms in each scene would show body language, so it might have been done on purpose :)
Okay, this isn't really a slip-up to me... I mean when a person talks to me I may stand in a thousand different "poses"...
Don't you move around a little when you are standing talking to someone?