Apple's Mountain Lion upgrade is available through the Mac App Store, and while most purchases are downloading just fine, a few people are experiencing a problem where the download will not complete. Instead of initiating a download, the Mac App Store issues a warning stating it could not complete the purchase because the product distribution file could not be verified, indicating damage to the file or an invalid digital signature.
People experiencing this problem have tried quitting and relaunching the Mac App Store, restarting their systems, and even trying Safe Mode, and have also tried waiting to see if the problem was a server-related issue on Apple's end that would be fixed; however, these efforts have not resulted in any change.
A few people have found that they could clear the issue by first uninstalling VirusBarrier, so if you are experiencing this problem and have VirusBarrier installed, you can try either uninstalling or temporarily disabling it to see if this allows the download to proceed. If you have other security software installed, then consider doing the same to see if you have any success.
Another approach that is working for people is to clear out caches and other temporary files used by the App Store. To do this, open the Terminal utility (in the /Applications/Utilities/ folder) and run the following command:
A Finder window will appear that shows a number of folders. Locate and remove the folder called "com.apple.appstore," and then try launching the store and purchasing Mountain Lion again.