Ariel Sharon, Prime Minister of Israel, cancelled plans to address Israeli support groups in New York City on September 11th.
This claim first materialised in a story entitled "4000 Israeli Employees in WTC Absent the Day of the Attack", promoted by Lebanons Al-Manar TV. Here's the part of the article relating to Sharon:
"Suspicions had increased further after Israeli newspaper Yadiot Ahranot revealed that the Shabak prevented Israeli premier Ariel Sharon from traveling to New York and particularly to the city's eastern coast to participate in a festival organized by the Zionist organizations in support of the "Israel"."
But what festival? Which organisations? When was he travelling? This all seems a little vague. That didn't prove a problem for many conspiracy sites, though, and it's now listed in many places as proof of foreknowledge regarding 9/11.
As usual, though, the truth is a little different. There was an event Sharon was supposed to be attending, held in New York, by the General Assembly of the United Jewish Communities, however this was to be held on the 23rd of September, not the 11th.
NEW YORK -- Via planes, trains and automobiles, tens of thousands of Jews from across North America are expected to descend upon New York on Sept. 23 for the long-anticipated continental rally in support of Israel, organizers say.
Preparations for the rally have begun to gel, with speakers and sponsors spanning the religious and political spectrum, including Israeli Prime Minister Ariel Sharon...
The UJC tentatively planned a demonstration for June 4 at Madison Square Garden in New York, but canceled for what officials said was an inability to guarantee Sharon's presence -- or his security...
In addition to Sharon, speakers from Israel will include Knesset Speaker Avraham Burg and perhaps Housing Minister Natan Sharansky. American speakers include New York Mayor Rudolph Giuliani and Gov. George Patak, and Sens. Charles Schumer (D-NY), Hillary Clinton (D-NY) and Joseph Lieberman (D-Conn), the former vice presidential candidate...
It seems the rally could have been held in June (safely), but was only put off until September by Sharon not able to guarantee he'd be there. This seems an odd kind of foreknowledge.
What's more, the rally itself was understandably cancelled on September 12th.
NEW YORK - September 12, 2001 - Respecting the need of all civilized people to grieve and begin healing from the horrific events of Tuesday, and in full support of law enforcement and public safety officials who are performing their duties under extreme conditions and emotional challenges, United Jewish Communities (UJC) and the Federations of North America will not hold the IsraelNow and Forever Solidarity Rally scheduled to take place in midtown Manhattan on September 23rd.
It's possible that after the attacks, but before the official cancellation, the Israeli Secret Service recommended Sharon shouldn't attend (it would be hard to imagine them doing anything less). Therefore it may be true that Sharon cancelled travel plans on September 11th, but this is in no sense mysterious, and doesn't require any foreknowledge at all.
See http://www.nocturne.org/~terry/wtc_4000_Israeli.html#NYC for more details on this and other Israel-related conspiracy theories.