Quote Originally Posted by Peelee View Post
Invulnerable. Not immortal. And she clearly thought she did and didn't consider the heel. Again, people, not robots.
So I edited that, but for the sake of convincing.

It is my experience that injuries to the feet, say, the heel, are the most likely childhood injuries. "One of Achilles' feet is vulnerable to harm" is something that honestly should have come up a long time before the Trojan War even started.

Furthermore, as noted, at least one translation has Thetis go beyond making him invulnerable and flat out try to make him Immortal by coating him in ambrosia and placing him in a Hearth to burn away his mortality and make him unable to die. In this version of the story, Pelus sees this and snatches newborn Achilles from the fire before it can have any meaningful effect... But Thetis explains herself to him and there's no reason given for why they don't just put him back in the fire once everything is cleared up.