Fallacious yet widespread and documented beliefs courtesy of Wikipedia.
During the German Invasion of Poland in 1939, there is no evidence of Polish Cavalry mounting a brave but futile charge against German tanks using lances and sabres. This seems to have its origins in German propaganda efforts following the Charge at Krojanty in which a Polish cavalry brigade surprised German infantry in the open and charged with sabres until driven off by armoured cars. While Polish cavalry still carried the sabre for such opportunities, they were trained to fight as highly mobile, dismounted infantry and issued with light anti-tank weapons.