The slip-up is in the episode where Ren and Nelson both fake sick to get out of their dinner party and go to the ice rink. In the beginning, Nelson asks Ren to give him a check-up on his puff-balls in his throat. When the camera view is from inside Nelson looking out, you see the bottom and top row of teeth across the top and bottom. But when the shot is in a normal view, Nelson's tongue is covering the bottom row of his teeth. This is not the biggest slip-up ever, I know.