Your challenge if you choose to accept it is to have Napoleon defeated earlier then he was IOTL with a POD of at the earliest 1799. Bonus points goes for a POD that defeats him during the height of his power. Extra bonus points if someone can change one of Napoleon's great victories into a defeat realistically.
Marengo (July 14 1800) actually was a defeat that got turned around at the last minute thanks to Desaix force marching his division back - the army under Napoleon's control was already in full retreat at the time. If the Austrians had been a bit faster or Desaix a bit slower then Napoleon's command would have disintegrated completely and his reputation with it. Heck, even if the battle had gone as OTL but Desaix survived instead of being killed in the fighting then Napoleon would not have been able to so shamelessly steal credit for the victory and use it to build his reputation.
Eylau (7/8 February 1807) - it's easiest just to quote wikipedia on this one. Referring to the aftermath of the disastrous first French attack on the second day -
"Bennigsen took full advantage; falling on Saint-Hilaire's division with more cavalry, and bringing up his reserve infantry to attack the devastated French centre. Augereau and the three or four thousand survivors fell back on Eylau, where they were attacked by about 5,000 Russian infantry.
At one point Napoléon himself, using the church tower as a command post, was nearly captured but members of his personal staff held the Russians off for just long enough to allow some battalions of the Guard to come up. Counter attacked by the Guard's bayonette charge and Bruyère's cavalry in their rear, the attacking Russian column was nearly destroyed."
My emphasis. Delete "nearly" and the Napoleonic era is over.