Yes that is pretty funny (and nnoticeable) but there is a logical reason as to why it's there. you see the armor he was wearing? normally if he fell on his back without that pillow his head would bend far back before his head hit the ground, and that might hurt a but!
Ok, now THAT is a little ridiculous. I understand the reasons why they had it there and that his head would have looked odd in the scene otherwise, but, I mean, come on, they could have found a way to cover it up better than they did.
Oh come on. They did that on purpose so I don't think it would stand as a slip-up. Also, like someone else said, the armor that he was wearing would cause his neck to bend back really far and it would hurt *a lot* so they put it there to make sure he doesn't get whiplash or something.