What was the greatest mistake of mrs loisel that became the cause of their miseries?

Question: What was the greatest mistake of mrs loisel that became the cause of their miseries?

One of the biggest mistakes Mrs. Loisel made to show off her mere beauty was the necklace. When she saw the necklace at Madame Forestier's house, she became very excited to wear an expensive necklace. He only thought of his beautiful victory. He never even thought about the ill-effects.