Maybe they just make this stuff up for fun

al-Jazeera.net has the purported story:

A US aircraft unintentionally violated Iranian airspace and was forced to land in Iran, an Iranian news agency says. The Fars news agency did not say when the incident happened or give a source for the report on Tuesday, which comes at a time of tension between Tehran and Washington over Iran's nuclear programme.
But the US defence department said it was unaware of any US military aircraft being forced to land in Iran and that all US aircraft in the Gulf region were accounted for. Lieutenant-Colonel Patrick Ryder said: "According to the combined air operations centre, all our aircraft are accounted for and we have no reports of any aircraft landing in Iran." Lieutenant Stephanie Murdock, a spokeswoman for the Fifth Fleet, told the AFP news agency from its base in Bahrain she had "no information right now" about the Iranian reports.
Fars reported that the jet was carrying five military officials and three civilians from Turkey to Afghanistan when it "unintentionally" entered Iranian airspace. Iranian fighters guided the jet to an Iranian airport, the passengers were questioned and a day later were released and allowed to continue to their destination, Fars said. It said the jet was a Falcon, apparently referring to a passenger aircraft manufactured by the French firm Dassault.
Al-Alam, Iran's official Arabic-language television station, quoted an unnamed military official as saying that the jet belonged to either a British or Hungarian relief agency. He said the aircraft "lost its way" and violated Iranian air space. He said the passengers included American military personnel but did not elaborate. "This plane was not a military plane and did not belong to the United States," the official was quoted as saying by Al-Alam. The aircraft was forced to land in Iran on Sunday and allowed to fly to Afghanistan on Monday, the website said.
Alireza Ronaghi, al Jazeera's correspondent in Tehran, said: "Apparently, it was a small passenger plane, carrying five high-ranking American generals and three citizens. The plane entered Iranian airspace via the Turkish border. Iran noticed it and forced it to land. After interrogations were completed, it was clear that they entered [Iranian airspace] unintentionally, with their destination being Afghanistan. We don't even know what day of the week this happened. [But] we know that they have been released and have continued journey to Afghanistan."

Rico says okay, it was a British plane, or a Hungarian plane, with five US generals on it, and it came and went either on Sunday or some other day... And CNN says the US says only some of it happened anyway:

The U.S. military coalition in Iraq confirmed Tuesday that a business jet -- not a U.S. military aircraft -- was recently forced down in Iran due to an airspace violation.
"The airplane is now being confirmed as a light transport plane with no Americans onboard," Multi-National Forces-Iraq said in a statement issued Tuesday. "From what we have been seeing, it was a Falcon business jet. We have accounted for all our aircraft and none are missing." The coalition in Iraq had no information on who owned the aircraft, stressing that it was not a registered American plane.
Fars reported that the 'Falcon fighter' -- which it said was also carrying three civilians -- entered Iranian airspace on Monday at a low altitude from Turkey to avoid radar detection, despite repeated warnings by the Islamic Republic Air Force.
National Security Council Spokesman Gordon Johndroe said there is no indication that the reports are accurate. "We're looking into the various and conflicting reports coming from the Iranian 'news' agencies, but do not have any information at this time that would lead us to believe they are correct," he said.

Rico says that the Iranians sound, to quote Lewis Black, confused...