When you purchase an app from the Mac App Store, it normally downloads and installs exactly as expected. But not always.
One error that has cropped up for numerous users over the past year states: “The product distribution file could not be verified. It may be damaged or was not signed.” When this error appears, the download of the purchased app fails.
The symptom seems more likely to occur if you are running Snow Leopard (OS X 10.6.x) than Lion (OS X 10.7.x), but it has been reported with both OS versions. A potential solution is to delete the App Store app itself from the Applications folder and reinstall the app. To do the reinstall for Snow Leopard, download and run a combo update (such as Mac OS X 10.6.8 Update Combo). An Apple Support Communities thread offers more details. If you’re running Lion, another thread confirms that reinstalling OS X via Lion Recovery should similarly work. I assume running the OS X Update 10.7.2 Client Combo would also succeed.
For those willing to take a more “techie” route to a solution, you can avoid a time-consuming reinstall. Instead, use the Console utility to identify the folder, located in the /var/folders directory, that is “failing.” A problem with the folder appears to precipitate the App Store symptom. The “corrupt” folder may vary (with names such as 3p or q1) but will be cited in the Console log. Once you’ve targeted the folder, delete it; the symptom should now be gone. A new folder will be created as needed. This works whether you’re running Snow Leopard or Lion.
TUAW recently covered another Mac App Store download error. This one states: “We could not complete your App Store request. An unknown error occurred (100).” In this case, the cause is apparently linked to a disconnect between the App Store app and the latest iTunes Terms and Conditions. You need to agree to these terms before a download can occur. The precise fix varies. It can be as simple as quitting and restarting the App Store and/or iTunes apps. You may also have to restart your Mac. If any of these restarts work, the Terms and Conditions window should appear prior to the next download attempt. After agreeing to the terms, all should be well. Otherwise, try downloading an iOS app from iTunes; this should bring up the Terms and Conditions. Agreeing to them should fix the problem for both the iTunes Store and Mac App Store.