Shelling of Mainila
The Shelling of Mainila was a military incident on November 26, 1939, during which the Red Army imitated shooting at the border village of Mainila, Russia while pretending that the shelling originated from Finland and claiming losses in personnel, thus getting a great propaganda bonus that launched the Winter War four days later.
According to the archives of Soviet party leader Andrei Zhdanov the entire incident was orchestrated in order to paint Finland as an aggressor and launch an offensive. The Finnish side disclaimed responsibility for the attacks and identified Soviet artillery as their source — indeed, at the time Finland didn't even have cannons that were able to shell the village from their side of the border.
However, in the days following the Shelling of Mainila, the Soviet propaganda machine generated publicity about other Finnish aggression (real or not), renounced the non-aggression treaty with Finland, and on November 30, 1939 launched the first offensives of the Winter War.
Recently after declassifying the related military documentation it was confirmed that the daily reports from troops dislocated in the area did not report any losses in personnel during the time period in question, thus proving the forgery.