Also, I am browsing the net since at least 12 years and i have NEVER found child porn by accident or whatsoever.
I don't know about now, but back in the days of usenet I certainly found it by accident on more than one occasion.
You used to have to play a.b.p.e roulette by dowloading several chunks, re-assembling, and uudecoding -- and lo and behold one day I found myself looking at this crap.
I doubt people stumble on it by accident very often, but I don't get the impression it's relegated solely to the 'dark' internet.
Judging by the sheer number of stories we see about this stuff, it apparently isn't all that difficult to find -- that or we have to assume that all people who go looking for this are hugely sophisticated technical people, and I'm just not convinced that's the case.
I for one hope I never have to see it again, and I can't imagine anybody whose job required them to look at it on a regular basis. I can only assume that would be a rather soul-numbing chore -- the police who track this stuff and any poor schmuck who is manually verifying for Google or someone sound like the worst job ever.