Waterloo, the final defeat of Napoleon by the joint Prussian and British forces was an incredibly well-orchestrated defense by the Duke of Wellington, who bore the brunt of the French assaults until the Prussian army arrived and finalized the defeat. But before the Prussians arrived Wellington drove off multiple charges of the French cavalry, defended Hougoumont all day, and held La Haye Sainte for most of the battle.
Mistakes were made on both sides, and at several points Napoleon probably could have routed the British Army and then turned to deal with the Prussian Army. Most importantly for Napoleon, his second in command was Marshal Ney, not nearly the same calibre officer as Marshal Davout, who was left in charge of Paris during this fateful campaign. Napoleon’s final move at Waterloo was to dispatch the Middle Guard, not the Old Guard, but a terrifying unit nonetheless, and one of his elite. The British Foot Guards broke this charge, prompting the disintegration of the French Army and end of Napoleon’s Hundred Days.
Spartan training was the stuff of legends in Ancient Hellas and from the days of Thermopylae and Plataea an aura of invincibility arose, not unlike that which Napoleon would enjoy. Before Thermopylae Spartans were feared, but not considered invincible (ironic that a defeat would do so much to further an aura of invincibility). Throughout the next hundred years or more Spartans were nigh undefeated on land, the major exception being on the island of Sphacteria off of Pylos, where a group of Spartans ignominiously surrendered. Then came Leuctra.
Thebes waxed while Sparta waned, became softer, more materialistic and lazy. In 371 the Spartan king Cleombrotus led the army up into Boeotia where a smaller force under Epaminondas caught it at a disadvantage and crushed it. In one fell swoop the Spartan predominance and invincibility disappeared.
In both situations armies were mismanaged and the loser could easily have emerged victorious; but in neither did they. Spartan hoplites could have held and the Imperial Guard could have kept advancing, but they didn’t. Both groups broke, both groups ran, auras of invincibility irreparably shattered. It took just one instance, one flight, one complete defeat.