I don't know really, certainly I can recall no specific incident. I suspect just the lessons in logic needed to learn to program computers properly, the basic lessons in the scientific method taught at school.
My folks are Christian, and I was still at Sunday School till I was about 14, but not really taking it seriously, still going just for the sake of a quiet life. By the time I was 18 at Uni I was certainly talking friends out of their theism, mostly by pointing out contradictions in their beliefs and challenging others to find some in mine. Then altering my beliefs when they occasionally found some.
Frankly my memory of my childhood is so appalling it'd be hard to have any confidence in a story if I told one.
Here we are nearly twenty years after that and drifting across a couple of OB posts from Eliezer through links elsewhere, probably during the QM series, maybe a bit before, led me to put OB into my RSS feed.
I'm always so far behind reading my RSS feed that feels like the discussion is over by the time I read anything there though, so never posted.
Pre..............
To break up the awkward silence at the start of a recent Overcoming Bias meetup, I asked everyone present to tell their rationalist origin story - a key event or fact that played a role in their first beginning to aspire to rationality. This worked surprisingly well (and I would recommend it for future meetups).
I think I've already told enough of my own origin story on Overcoming Bias: how I was digging in my parents' yard as a kid and found a tarnished silver amulet inscribed with Bayes's Theorem, and how I wore it to bed that night and dreamed of a woman in white, holding an ancient leather-bound book called Judgment Under Uncertainty: Heuristics and Biases (eds. D. Kahneman, P. Slovic, and A. Tversky, 1982)... but there's no need to go into that again.
So, seriously... how did you originally go down that road?
Added: For some odd reason, many of the commenters here seem to have had a single experience in common - namely, at some point, encountering Overcoming Bias... But I'm especially interested in what it takes to get the transition started - crossing the first divide. This would be very valuable knowledge if it can be generalized. If that did happen at OB, please try to specify what was the crucial "Aha!" insight (down to the specific post if possible).