What lesson did Hercules fail to learn?

Hercules, the son of Zeus and Alcmene. He was born in Thebes, Greece.

His name means "Glory of Hera". Yet Hera was his arch enemy.

His missions were 10/12 by Oracle of Delphi

1. Conquer and deliver the Nemean Lion, a monstrous feline.

2. Kill the multiheaded Hydra.

3. Bring back, dead or alive, the Cerynitian Hind, a ravaging deer.

4. Catch the Erymanthian Boar.

5. Clean out the massive stables of Augeas, perhaps the most famous of the Labors.

6. Scare off and kill the metal-feathered Stymphalian birds.

7. Capture the Cretan Bull.

8. Do something about those pesky man-eating Mares of Diomedes (he moved them and released them.)

9. Get the Girdle of Hippolyta, Queen of the Amazons (she gave it to him peacefully, which enraged Hera, who arranged for the rest of the Amazons to attack Hercules; in the mess that followed, Hippolyta was killed by Hercules.)

10. Steal the cattle of Geryon.

11. Bring back the Golden Apples of the Hesperides.

12. Go down to the Underworld and bring back multi-headed Cerberus, chief Hound of Hades.

How did Hercules die?

When Hercules was trying to take his bride Deianeira home, he had to cross the Evenus River. Nessus, a centaur, served as ferryman. First he rowed Hercules across and then, as he started to row Deianeira across, he tried to rape her. Hercules drew one of his poisoned arrows and shot the centaur. Before he died, the centaur persuaded Deinaeira to take some of his blood to use as a love potion should Hercules ever cause her to worry.

In time, Deianeira became suspicious of Hercules' interest in another woman, named Iole, so she smeared some of the carefully-saved centaur blood on a tunic and gave it to Hercules, trusting that it would act as a love potion and return him to her.

Unfortunately, the centaur had lied. The blood contained not a love potion, but a powerful poison from the poison with which Hercules had tipped his arrows. It had come from the Lernaean hydra.

When Hercules put on the tunic, it burned. He was in such excruciating pain that he wanted to die and had a funeral pyre built for himself. He then mounted it and had it lit. He died and went to the gods where he was reconciled with his tormenter, the queen of the gods, Hera. She allowed him to marry her daughter Hebe and live among the gods thereafter.