Can You "Possess" a File If You Don't Realize It Exists?:
One of the interesting questions raised by the application of traditional contraband concepts to digital files is whether a suspect who views a contraband image using a web broswer but does not realize the image is stored in the browser cache "possesses" that image based only on the presence of the file in the cache. In a decision handed down today, United States v. Kuchinski, involving charges of possessing child pornography images, the Ninth Circuit concluded that the answer is "no":
Where a defendant lacks knowledge about the cache files, and concomitantly lacks access to and control over those files, it is not proper to charge him with possession and control of the child pornography images located in those files, without some other indication of dominion and control over the images. To do so turns abysmal ignorance into knowledge and a less than valetudinarian grasp into dominion and control.
  I think this is correct as a matter of doctrine. If you don't know an image is there, you can't possess it. In most cases this isn't an issue: a suspect who seeks out an image and knowingly retrieves it will be guilty of knowing receipt, and there will usually be some evidence of dominion and control other than presence in the browser cache. But it seems to me that mere presence of a file in the browser cache shouldn't be enough to establish possession of the file. (For more on this issue, see my casebook, pages 224-32.)

  Thanks to Howard for the link.