I believe what I said was at times, pretty much most of the characters look off-model at times, not just Arthur himself. Like someone else previously pointed out, so far, Francine and Muffy seem to be the biggest victims of this; I'm not entirely sure just how, exactly and specifically, they're animating the show (again, I've animated in Flash before, so I know there's different ways of going about it), but I know most of the time, Flash can "auto fix" what it perceives to be mistakes, so I wonder if them trying to draw Francine and Muffy's faces the way they traditionally were were seen as errors in Flash that kept being fixed.

I really can't name any specific moments right now without looking at some of the episodes again, but as I recall, Arthur and D.W. both have had moments where they look just slightly off model, but again, Francine and Muffy stick out the most.

I didn't really notice it much for Muffy, but for some reason, as I said earlier on this topic, Francine's nose, when she is seen from the side, looks a little big and round. As for Arthur, I think it's like either his glasses/eyes are too big, or the spacing of his mouth, nose, head, ears (basically all of his facial features) are a bit off.

BTW, earlier I remember you said that D.W. looked like she had "sharp edges" in certain scenes. What do you mean by that?