Like Gordon said, occasionally an extra cuff or two, maybe a pistol, never anything big.
With tiny plastic bits it wouldn't be worth unpacking everything to retrieve a cuff.
What I wonder at is the bagging process. Unlike Lego, where bags are numbered and all the parts for a particular step ate together, you often have to open several bags to get even one part of a set assembled. Presumably this is linked to the weighing process, or maybe the different parts come from different areas of the factory.