Nouriel Roubini, aka Dr. Doom, who called the crash correctly, is warning of another, larger one. Here's the short version.
The Fed is keeping interest rates low, is printing money to buy US debt and the dollar is getting killed. The end result is that global investors borrow dollars and then buy foreign securities, US stocks and other risky assets with those dollars. They make money just on the fact that the US interest rates are low and if they go foreign with it, the falling dollar makes more money for them. The result is a megabubble. As soon as interest rates rise and the Fed stops printing money, the flow reverses and everyone will rush to sell their investments. CRASH!
At least that's the way Nouriel sees it. The long version is here. I had to read through it twice to get the idea.