We tend to think of our memories as being something fixed and solid. In reality, memory is fluid and often very unreliable. We tend to remember what we want, what makes sense, and what makes use conform to our ideals about ourselves.
In one experiment, people were shown two photographs and asked which one was more attractive. Then through a slight of hand trick they were given the photograph that they had not chosen–the one they had decided was less attractive. They were then asked to explain why they felt that photo was more attractive.
74% didn’t notice the switch. Worse they came up with a bunch of reasons why they had chosen that photograph. Their memories basically switched to make them believe that the photo they had been handed was the one they had chosen.