The brewpub in Artesia gave me a bag of Briess two row once because the brewer found metal chunks in it. It was a seized bearing, probably from a conveyor belt used to fill the sack. I just poured the malt over a big magnet before I poured it into the mill and got all the pieces out, there really wasn't very much.
Otherwise just the random crap that is associated with any agricultural product-twigs, seeds of other plants, etc.