The crew of a Boeing 747 flying from Alaska to South Korea managed to mis-configure their planes systems and over the course of the flight the autopilot drifted off course towards and into the USSR. As it entered Soviet airspace it crossed paths with a US spy plane that had been doing reconissense on the USSR, to the Soviets the radar track appeared to show the airliner turning away from their airspace and the spy plane heading towards it. The Soviets then lost track of the airliner and didn't see it approaching their territory until it was practically over it because the radar that would have lit it up wasn't working (the local officials had lied to Moscow that it had been repaired following weather damage when in actually it was still non-functional) they then scrambled interceptors to intercept it.
Due to it's speed and heading the airliner was able to cross Soviet territory and get away back over international waters before the interceptors could reach it (you had one job interceptors lol). Sadly because the plane was flying a fairly predictable course the Soviets knew it would cross their airspace/land again as part of it's course and had another interceptor waiting to intercept it if it violated their airspace again. The commander of the air defences ordered that if it escaped over international waters it could only be shot down it it was positively identified as a spy plane (escaping with it's reconissense) however the sector commander below him instead ordered for it to be shot down if it flew near their airbase (no requirement for identification). When the airliner did indeed violate their airspace again and fly over the USSR the interceptors (four of them) visually identified it however none of them bothered to mention that it had airliner lights (concluding that an airliner derived spy plane could easily have them too). One of the interceptors was then ordered to meet it and attempted to communicate with it to force it to land, first by flashing his warning lights then when that failed he tried firing warning shots which (according to the cockpit voice recorder) they didn't notice as there were no tracer rounds.
It was at that point that disaster struck, immediately after the warning shots were fired the crew of the airliner pulled up to gain altitude, this caused the interceptor to slingshot past (it's Maverick's move from Top Gun). The interceptor believing it to be a deliberate evasive manoeuvre, the pilot notified his command who gave him the order to shoot it down, he then pulled around for a shot and launched missiles crippling the airliner. The interceptor pilot then notified his superiors the target had been destroyed, in reality it stayed airborne and the crew managed to maintain control for a while until too much of the plane had disintegrated and it spiralled into a crash.
In the aftermath the USSR lied about pretty much everything, what happened, where the plane was, who was responsible, etc etc. To this day the interceptor pilot believes he shot down a spy plane.