I co-authored an article on this topic in 2004 that was published in the Santa Clara Law & Technology Journal. Back then, there was only one case where this defense was used and that was in a hacking case. I hear some version of this defense quite often and I have to always questions whether it is legitimate or not. Someone thinking about using this defense will often say that they clicked on something and a bunch of pop-ups came up and stuff started to download on their computer. This rarely pans out as the defendant doesn’t realize that it is often fairly easy to show how the child pornography got on the computer and chances are someone (often them) put it there.
However, the case of Michael Fiala proved that this could be a possible defense. Of course, in order to have such a defense, you need to have very sloppy police work. Thankfully, the agencies I have dealt with would have likely picked up on this and thus, the defendant would have never been prosecuted to begin with.