Madame Loisel has many flaws, but the two most obvious are 1)her greed or lust for material things and 2) her inability to confess the truth about what happened to the necklace.
Pride is also a huge fault which plays into both of these reasons. Her pride makes her feel that she is entitled to the "good" life of balls, gowns, fancy jewelry, and servants to wait on her every whim. It is also the reason that after the night of her life, she can not bring herself to admit to her school friend, Madame Forestier, that she has lost the necklace which Madame Loisel borrowed.
Because of her greed, she borrowed a necklace to add sparkle to her attire for the ball. The pride has kept her from confessing the truth, and has forced her into a decade of hard labor and debt in order to replace the cherished jewels. She has also lost her youth and beauty due to her pride and refusal to admit the truth.
Her friend, Madame Forestier, is still young and lovely and barely recognizes Madame Loisel years later when they meet again. Upon hearing the story, Madame Forestier confesses that the necklace was only paste...not real at all. Had Madame Loisel told her friend the truth, she could have saved her husband and herself much suffering.