Do you have a citation for that, out of interest.
It's considered something of a myth within LE and I wasn't aware a case like that actually existed.
I also recall a case of a teacher whose school computer had malware that ended up displaying porn in the classroom who was arrested and faced up to 40 years in jail. I'm pretty sure she was acquitted.
After the initial conviction was set aside, after 4 delays in sentencing, "[...] facing health problems, Amero pleaded guilty to one count of misdemeanor disorderly conduct, paying a $100 fine and losing her teaching license, just to settle the case."
I'd always be interested to see if it actually happened though - it would be useful research.
Mostly it is very easy to see the CP activity (dont forget were not just talking about images on the HDD here but actual activity indicative of behavior). If it were downloaded all together, with no references in, say, web history etc, no thumbnails and nothing in the cache files (which are FULL of rich information) then it would ring alarm bells :)
Eventually even the most elaborate malware will make a mistake with activity that is not "normal" - and the investigation is intensive enough to pick this up.