Date of prediction: 2006-07

In 2006, Foreign Policy magazine interviewed 116 experts on the likelihood of major terrorist attacks over the next five years:

»The Very People were asked to put forward their considered opinions about how likely it was that “a terrorist attack on the scale of 9/11” would again occur in the United States by the end of 2011—that is, by last Saturday.

Fully 70 percent found it likely and another 9 percent proclaimed it to be certain. Only 21 percent, correctly as we now know, considered it unlikely.«

(The National Interest:
New Year Brings Good News on Terrorism: Experts Wrong Again, via Schneier on Security)