Pity poor David Lipin. He cashed a $1000 USPS Money Order at a local post office and got a wad of counterfeit bills, according to the L.A. Times. He didn't know they were fake; and presumably the post office clerk didn't know they were fake. But the gas station attendant on Lipin's next stop immediately did -- and called police.
Now Lipin is out a grand, even though it was handed to him by a government institution. The Times quotes Wayne Williams, who oversees the L.A. office of the Secret Service (which is part of the Treasury): "The post office operates as a business. It takes in money from customers. Postal workers don't really have special equipment or training to spot fake money." And he's talking about government employees here.
Moral of the story? Well, there's several. Most obviously, even the experts can be fooled. The guilty post office failed at least two chances to catch the forged bills -- once when taking them in and then again when handing them out.
More importantly, the government seems to put the burden of false money on the accepting party. Quoting Williams again, "Unfortunately, counterfeit money is like a hot potato. Whoever ends up with it last is the victim." It is your responsibility as a business, as an individual consumer, to ensure the cash is real. The buck does literally stop with you.