I was shown the movie Zeitgeist on my mission. It took about a minute to realize that it was "anti" material, and I was a bit annoyed with the guy showing it to us, but my curiosity got the better of me, as usual. It was very compelling. My comp was not a strong English speaker, so he thought the movie was about Easter or something (because it was April, I guess).
The movie didn't bother me at first, and I was proud of myself for having such an impenetrable testimony. I even joked about "getting anti-ed" with my ZLs that night. But the very next morning cognitive dissonance hit me like a truck. By lunch time it was unbearable, and I decided to get some resolution. I tested my faith. I imagined that there was no god. That's the best way I can explain it. It was a very intuitive experience. I remember imagining that the thoughts in my head were solely mine and that the picture of Jesus on my desk was a depiction of a fictional character. I thought about the world and my own personal experiences, and wondered if a god was needed to make it all work. Then I had the most sickening paradigm shift of my life. The picture of Jesus became meaningless, and I felt very alone in my own head.
I was afraid that I had lost my testimony or offended god. But I was just being honest! If god created me, he knows how my brain works, he knows the thoughts in my head, and he knows I'm doing the best I can with the information I have. If he chooses to punish me for my honesty, then he's either spiteful or doesn't exist. That was the moment I became an atheist.
Met with my mission president a few days later. His testimony consisted of the compelling argument, "I've read the BoM so many times, I just can't imagine it not being true!" I stuck it out another week, read the entire D&C and BoM (which only solidified my decision to leave), then got on a plane another week later. Couldn't take the name tag off fast enough (I actually "forgot" to put it back on after going through security). Couldn't get online to do research fast enough. Thanks, MormonThink!
P.S. I know Zeitgeist contains a large amount of fabrications. I researched that, too.