I have read the court papers on the case you refer to (as a part of a computer law course). The guy had set a program to download some newsgroups full of (legal) porn, and he discovered later that some of the pics were of children. He had deleted the ones he found and later testified that he hadn't been sure if he'd gotten them all (how could he, he can't check the age of everyone in a huge amount of pics). The court reasoned that even though they agreed the man's possession of child pornography wasn't intentional, he must have thought it possible that everything on his hard drive wasn't legal and hence he was found guilty.
As I recall, there was also a weird twist in the case where the police confiscated his hard drive to use as evidence, but after compiling a list of child porn on it they somehow managed to lose the original and all the copies, so the defense couldn't use it in court.