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!