Lord of the Rings: The Return of the King - Disappearing Crumbs
When Smeagol throws the Elven bread off the steps, and him, Sam, and Frodo are trying to figure out who took it, before Smeagol shows Frodo the crumbs on Sam, there's a close up of Sam and you can clearly see there are no crumbs on either shoulder. A few seconds later, the same shoulder which had no crumbs on it, is now covered with crumbs.
Well, this is, and isnt a slip-up. It is because you cant see the crums on his jacket, like you said, but then again, if you could see the crums, dont you think eaither he would notice them and wipe them off or Frodo would notice them and say something? That was a wierd part. Good cetch. :)
The thing that I don't really understand is that the crumbs are on his shoulder, when you would eat facing ahead. I suppose it's possible that he was looking to the side, but it's strange nonetheless.