Napoleon's first mistake was "The Continental System". What the Continental System was was a blockade of British Ports and it forbade all nations trade with Britain. It ended up failing because the restrictions of trade were lifted and Britain took advantage of it. Which caused the plan to backfire on Napoleon.
Napoleon's second mistake was "The Peninsular Campaign". The purpose of the campaign was to capture Richmond. The reason this ended up failing was his army wasn't ready for the confederates artillery.
Napoleon's Third and final mistake was the "Invasion of Russia". The invasion of Russia was Napoleon and his men invaded Russia and stayed in Moscow for 5 weeks. This ended up failing because he had faulty logistics, poor discipline, and disease.