I was recently trying to upgrade to mountain lion through the app store. I have unreliable Internet from a cable company whom I will not name, but we all know who they are. My Internet dropped while downloading the upgrade. Once I went through the notorious unplug the modem/router and plugged it back in to gain Internet connectivity again, I could not resume. No matter what I did, the app store kept displaying "an error occurred." I am hoping that those with the same issue read this first to save them hours of skimming through the Internet to find a solution. Here are the steps to take:
1. Make sure you have reestablished Internet connection
2. Chances are you got frustrated when you tried to resume/unpause the download and clicked the "X" to the left. If so, select the "Store" drop-down menu from the App Store and select "View My Account." There should be an option to unhide your purchase. You know what to do.
3. There is also an option to reset all warnings for buying and downloading under "View My Account." Do this, click "Done," sign out of the App Store and quit the App Store.
4. Open the Preferences in safari and select Privacy. Select "Remove All Website Data." Just do it.
5. Launch the app store and sign in.
6. Now, if you "Check for Unfinished Downloads" everything works peachy.
I hope this helps. Not much I can do for your Internet connection though.