Well, this is horrifying. Street vendors in China have taken to using something called "gutter oil"—quite literally the oil gathered from gutter runoff, dumpster sludge, garbage juice, and untreated fucking sewage fuck fuck fuck—to cook food for human consumption. And then unwitting humans are consuming that food. As you might guess, not everyone's happy about that.
This paragraph is gonna be really gross. So, evidently, the way it works is, gutter-oil-makers root around in dumpsters, trashcans, sewers oh God sewers, and (of course) gutters, looking for used oil or discarded food or solid waste or dead fucking animals, and then they process that putrid crud (this probably entails heating it to render any fats into oil, then straining out the solids) and sell the resulting distillation of pure horror as a cooking agent. Then street vendors use it to cook what is presumably already F-grade meat. Then people purchase and eat the "food" cooked in this effluvium of evil. Then you read about it on the internet and become a raw-food evangelist.
Chinese authorities busted a black-market gutter-oil operation back in April, seizing 3,200 tons of gutter oil which had been rendered from rotten animal carcasses. By the time of the sting, the operation had already sold an estimated $1.6 million in carcass-juice. To food vendors. For cooking food in it.
Hilariously, the author of the linked Washington Post article advises that you shouldn't "cancel any vacation plans to China over this." Well, sure. I suppose if you weren't troubled by China's human rights record, or the suffocating pollution in its hellish cities, or the abominable conditions in which China's workers toil for the sake of pumping cheap consumer goods into the West, the possibility that your friggin' dumplings have been fried in rendered feces and roadkill probably shouldn't be that big a deal. Bon voyage!