We'll know in September. From the Wall Street Journal:

Forget about Friday the 13th. Many on Wall Street took to whispering about an even scarier phenomenon—the "Hindenburg Omen."

The Omen, named after the famous German airship in 1937 that crashed in Lakehurst, N.J., is a technical indicator that foreshadows not just a bear market but a stock-market crash. Its creator, a blind mathematician named Jim Miekka, said his indicator is now predicting a market meltdown in September.

Wall Street has been abuzz about whether the Hindenburg Omen will come to bear, with some traders cautioning clients about the indicator and blogs pondering all the doom and gloom.

There's a technical but still accessible explanation of it on Zero Hedge. I confess I haven't heard of it, but apparently the fact that it's "easily the most feared technical pattern in all of chartism," combined with the fact that warnings about it came out on Friday the 13th, provides something for everyone to worry about.