For me it was a few different things. The first clue about Santa not being real was when my dad became diabetic. That year we put out a glass of diet lemonade for Santa. I found that a little suspicious.
Years later I discovered that teachers didn't know everything. I had assumed they did. At the age of about 11 or 12, I took to school with me a digital calculator that I had converted to a stopwatch (
scroll down to project 534). It involved buying a calculator and a bag of parts, including a PCB and some ICs, resistors, capacitors etc, soldering the components on to the PCB (well, you know, the leads), prising the calculator open, fitting the new PCB in, drilling a hole in the side for a push-button switch, wiring the new circuitry into the existing circuit, putting it all back together again. The teachers thought I was a freak. This was my first year of high school. I learnt that they had nothing to teach me, and any learning I had to do, I had to do on my own. Probably explains why I'm now a dysfunctional bum.
I also discovered that
all adults didn't know everything.
A couple of years later I discovered that most adults didn't know anything. (OK, I'm exaggerating just a little) Like, I'd be in my local Tandy's or Dick Smith's (think, Radio Shack) and asking the guy behind the counter what value resistor to use with this particular LED or something equally simple and he didn't have a clue. I'd ask another guy about thermal coefficients of the different heatsinks they had, again, no clue. I'd ask if there was a CMOS equivalent of some particular 7400 series TTL chip and they'd just stare at me. Up until then, I thought that people who worked in shops knew about what they were selling. I learnt that actually they were salespeople, not technically minded at all. I was so disillusioned.
Poor me. Boo hoo. The world did not meet my expectations. Waa waa. I got over it soon enough, although I'm still shocked to discover all the things that people believe in.