I've found it funny when I've made arguments about Google's ad scanning being something I didn't like, and people always came back with "but it's 100% automated and completely anonymous - no human ever looks at your mail".
I think that argument just got settled with this story - and I won.
No you didn't. If you had bothered to read the article, you would have seen that they detect things like this by using image hashing. It's an automatic process - unless you happen to be passing around images that are identical to known images of child pornography, at which point of course humans will get involved.
I really need to know more about whether this email triggered a thorough and careful investigation that led to the arrest of the person, or if the email WAS the trigger for his arrest.
Well, if you really need to know, then you could always read the article. It specifically states that he was arrested after police found other suspicious images on his computer (after obtaining a search warrant), and that he's a registered sex offender. Chances of this being a mistake are practically nil. All indications are that both Google and the police did their job properly, with judicial oversight.