http://en.wikipedia.org/wiki/Iran_Air_Flight_655

The US excuse is the standard bureaucratic bullshit, slight variations on the excuses for bombing the Chinese news agency and killing 400 civilians in a well-known air raid shelter. Always understandable, excusable, simply an oversight by nice guys doing their jobs competently.

But God help the Russians, evil bastards that they are

"According to the Iranian government, Vincennes negligently shot down the civilian aircraft: the airliner was making IFF squawks in Mode III (not Mode II used by Iranian military planes), a signal that identified it as a civilian craft, and operators of Vincennes mistook for Mode II.[4]
"According to the United States Government, the crew incorrectly identified the Iranian Airbus A300 as an attacking F-14A Tomcat fighter, a plane made in the United States and operated at that time by only two forces worldwide, the United States Navy and the Islamic Republic of Iran Air Force. The Iranian F-14s had been supplied by manufacturer Grumman in an air-to-air configuration only and had no known anti-ship capabilities.[5][6]"