No, we're not talking about Genghis Khan's crew. The Golden Horde is a term used on some preparedness boards for the mass of evacuees from major population centers once the SHTF. Think of all the resources that a city uses in a typical day, plus the gasoline needed to fill the tank of most every privately owned car in the place. Now imagine trying to fill that need with the contents of small town convenience stores within a hundred mile radius of that city. You no doubt heard of shortages along the evacuation routes back in '05 for Katrina & Rita, and again last week for Gustav. Ok, imagine that instead of New Orleans or Houston, it's every city in the US that's being emptied. Now add a mushroom cloud, UFOs, or a mob of zombies in their rear-view mirrors and watch the road rage turn lethal. That convenience store's last box of diapers could end up the subject of bidding, deal making, or outright violence.
It's not just upstanding suburban citizens in this mass migration. You're also going to be having all manner of thugs and junkies fleeing whatever it is that's being fled from. And even if you aren't one, you can sure that you'll be suspected of being such if you fit the profile in the locals' minds. During such an evacuation, double any negative reaction penalties that characters might have, if they're the sort that the locals might notice. (Not so much for appearance, unless it's specified as "creepy". Very definitely for things like low Social Status or being a member of a Minority Group.)
This sort of encounter works anyplace you've got population centers, hinterlands, and something to cause the former to empty into the latter. Perhaps the semi-obscure smuggling port your characters operate from could be unexpectedly filled to the brim with refugees fleeing a shift in the battle lines. Now you've got to get that questionable cargo from the storage facility to the docks without running afoul of either the enhanced security presence or the desperate refugees. In the fifties or sixties, flying saucers or giant radioactive dinosaurs could be curb-stomping the cities. A rouge nano-weapon could send streetwise cyberpunks scrambling to the sticks. The Dark Knight had the threats of a facially scarred madman emptying out Gotham. A minor "Ooops" on the part of the Mage Guild might force the thieves & merchants of a city into the countryside, as could a steampunk mad scientist's latest creation.
It could also factor into a character's backhistory, either as a victimized local, or an ordinary citizen who had to do unthinkable things to escape from Doomedville after the traffic ground to a halt.
UPDATE: A post from Bayou Renaissance Man on real world examples of the Golden Horde during Katrina & Rita.