In Greek mythology, Achilles's mother dips him in a river that protects his body wherever it touches. His heel does not get wet, so it is the one part of his body left unprotected. During the Trojan War, an arrow hits Achilles in the heel and kills him.
What is the meaning of the allusion in the sentence below?
Red velvet cupcakes were Stacy's Achilles's heel when she was trying to eat more healthily.