Case Status: Dismissed. It turns out that Thetis put Achilles in the fire to burn his mortal parts away. If Peleus hadn't interfered, then Achilles would've been totally invincible. Thetis was so mad at Peleus that she abandoned him, and Achilles ended up being raised by Chiron the Centaur. (Take a peek into Achilles' life with Chiron.)
Addendum: Other rumors have circulated that Thetis was dipping Achilles in the River Styx where Peleus may or may not have interfered. We've had reports that Achilles' heel became his famous weak spot, because his mother held him by his heel, not dipping him all the way into the sacred water.