Courtesy of The Urethane Blog, a US flour company (Wegman's) is recalling bags of flour because they may contain "blue polyurethane balls". I'm not sure what size the balls are, and consequently how noticeable they are. The balls are used in the sifting process, so I'm guessing that the mesh that was supposed to retain them broke and let them pass.
I doubt any serious harm would come of this contamination unless the balls are so small that they could be baked into say, a batch of brownies without anyone noticing, and then some unsuspecting person chips a tooth on them. Swallowing them would likely lead to no harm in most cases (they would pass through the GI track in short order and with minimal hydrolytic degradation), and there would be no chance of them leaching anything into the flour (they are already approved for direct food contact in order to be used in the sifting process), but nonetheless, they don't belong there and so the recall is appropriate.
The irony here, is that while the FDA doesn't allow these contaminants into wheat flour, they do allow up to 75 insect parts and 1 rodent hair in a 50 gram sample. Bon appetit!