When you buy your black-oil sunflower seed from a reputable wild bird supply store, you are usually assured of getting a good quality product, clean and free of noxious weed seeds and other contaminants. Your small shop owner depends on your return business and your recommendations to bird-feeding friends to stay open, so s/he is careful when choosing what suppliers to buy from. I might see the occasional grain of corn or striped sunflower seed, but in general, I am very happy with the black-oil seeds I get from my local store.
Even the best manufacturers sometimes have a problem with their source for sunflower seeds, and weed seeds may slip into the mix. The thing I most hate to find in my sunflower is this:

The seeds of Field Bindweed, also called Wild Morning Glory.
As far as I can tell, nothing eats this seed, so it ends up in your yard, infesting your flower beds and getting tangled in your shrubs and bushes. I try to be alert and pluck out any bindweed seeds I find, as a preemptive strike.
Edit, 12/23, 6pm: OOPS!! Donald tells me I have mislabeled the white seeds. They are not bad bindweed seeds, they are plain old soybeans. I plead ignorance. Somebody once told me they were bindweed, and I believed them. Thanks for the correction!