Freud would say that the mistake came from her unconscious mind. Sigmund Freud strongly believes that the unconscious mind governs a person's behavior even without us knowing. It is within the unconscious mind where our inner desires lie.
On the otherhand, cognitive researchers would say that it is a result of our brain or literally our cognitive side. This includes perception, learning and memory. This is different than unconscious mind because the person has knowingly accumulated these knowledge in the brain. Whereas the unconscious mind is still a great mystery to humans.