Fret not – these errors can definitely be fixed, taking just a few minutes of your life to rectify before everything ends up as plain sailing again. The easy way would be to quit the App Store by signing out of it, restart your Mac and login to the App Store before making another download attempt, but if this does not work, there is a more advanced way of going about it.
This time round, not only do you need to quit the App Store as mentioned above, but you will also need to delete prior “Install OS X Mountain Lion” installers – something developers would have on their machines, or those who have already tested OS X Mountain Lion prior to its release. Following that, empty the trash (force empty it if you need to) and resume the downloading process. Which method worked for you if you were affected?