If you happened to rush off the the Mac App Store to download Mountain Lion, be aware that thousands of others are doing the same…
Some users are reporting an error message when trying to download Mountain Lion. This may just be due to the high volume of downloads Apple’s servers are dealing with. Just be patient and things will smooth out soon.
Once the day has settled down a bit, you should no longer have issues with the download. Just keep on trying throughout the day.
Everything downloading okay for you? Meet me in the comments.