It's not impossible for SAM sites to incorrectly set a target. During the Gulf War an RAF Tornado was shot down by a Patriot missile that had incorrectly target

It's not impossible for SAM sites to incorrectly set a target. During the Gulf War an RAF Tornado was shot down by a Patriot missile that had incorrectly targeted the aircraft. Not long after, a US F-16 pilot reported he was targeted by a Patriot site as well, but the missile was not launched. In 2003 an F-18 was shot down by a Patriot, killing the pilot. Most countries have their own IFF (Identify Friend or Foe) codes in their transponders. Civilian planes also have transponders that identify them as non-threats, but that is not fail proof (MH17, the 1988 Iranian incident, etc).