Nancy has three fatal flaws.
Her ability to care about people who treat her like crap. Nancy is a caring soul- maybe because she was raised to be that way or maybe because it is the way that she is naturally. She cares too much about the people who try to harm her and unless you get on her bad side immediately, she will probably care about you until her dying day. It's just the way that she is.
Her ability to trust. Nancy is foolishly trusting. She is the lamb to everyone else's lion. She's been trying to work on it, she really has but even though she gets herself into a lot of trouble because of how much she trusts people, her first flaw prevents her from ever truly overcoming her second. This does not make her stupid- she is still a shrewd individual, it just means that she is more likely to trust someone if they tell her to trust them.
Her curiosity. Curiosity killed the cat and the cat can not be brought back to life. Nancy has learned this many times over but she still manages to get herself into trouble because of how curious she tends to be.