Orin Kerr over at The Volokh Conspiracy 

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":

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