I've known this for years, but some still don't believe: it's perfectly possible for your inexpensive dog food to contain the rendered remains of cats and dogs, among other nasty stuff. But of course - where did you think the pentobarbital found in dog food comes from, exactly?
It's not uncommon for high-kill shelters and municipalities that pick up a lot of roadkill to sell the bodies to renderers, and for years the rose-tinted glasses set has insisted that the products that result would never, ever be sold in the US... right?