Have you ever known someone, who turned out not to be the way you thought they were?

Have you ever known somebody, maybe a close friend, a schoolmate, or a co-worker, who presented themselves to be a certain way, and then later, maybe even years later, you found out that they were not at all like you thought they were? Maybe they did something really awful, or maybe they just made up a bunch of crap.

Stories are welcome.

Also, how did the ultimate knowledge, of how they really were, make you feel? What did you do when you found out the truth?