Accident is one of the better defenses to child pornography cases. This defense assumes either a favorable statement or no statement given by the defendant. Here, the defense will admit that the files are child pornography and that the defendant is responsible for possessing them. However, the defendant did not knowingly possess the files because they were acquired by accident. Keep in mind that this defense will be hampered by the amount of child pornography files as well as the different number of dates that the files were downloaded.
Common defense themes include the following scenarios: the defendant downloaded a large number of pornography videos and some were child porn, in other words, he cast a wide net and caught some bad fish; the defendant was searching for music or movies and the files were mislabeled (of course, they should all be mislabeled); or the defendant was searching for adult pornography and some of the search terms were contained in a child pornography file with a large name. Of course, there are issues that have to be tackled with all of these defenses in order to perfect them. For example, if the defense is focused on the search term, all of the child pornography files should have the exact same search term and there should be adult pornography with that same search term.
Also keep in mind that this defense may open the door to 404(b) evidence that would otherwise be inadmissible.