I remember hearing about virtual economies where money can be injected at any time through gold mining, etc, and that they were inherently unstable. Once you got the gold farming schemes that sold gold on e-bay, it created massive hyperinflation.
And it totally made sense that hyperinflation would occur since the creation of virtual gold was limitless and effortless, so the gold farmers didn't care how much gold they sold it for as long as they got real money. That rendered the price of gold worthless, and it drove up the prices of virtual goods across the board.
I would love to see how these problems are tackled in the virtual world.