Can a bag of "Potting Soil" bought at a Garden Store accidently have "Weed Seeds" in the Potting Soil itself?

Actually the cheaper brands can and often do have weed seeds in them since they are not always screened out when the "soil" is being mixed or packaged for sale. One reason I mix my own soil less mix here for our nursery use. Plus on our scale its cheaper for us and we get the exact mix we need for each type of plant or use! Oh and never buy "top soil" as its actually clay and almost always has weed seeds and other harmful things in it to start with. Good luck.