UPDATED: Serious Time Machine bug on Mountain Lion
This is continued and summarized from a previous thread, Serious Time Machine bug on Mountain Lion, which has grown rather large and unwieldy (and which I marked Solved in error). There are a few other threads that touch on it as well.
Some of us have seen a few cases of a pretty nasty problem with Time Machine backups on Mountain Lion. It doesn't seem to affect a lot of users, but those who have it, don't know it. It seems to have started at different times for different users, some as long ago as December, 2012.
The backups cannot not be used to do a full system restore from recent backups -- they don't appear on the selection screen that shows only "complete" backups. Plus, they're not recognized as backups by Setup Assistant or Migration Assistant. They don't appear on the Select the Source or Select Your System windows at all. But your user data is fine via the Time Machine browser (the "Star Wars" display).
What's happening is, the top-level Applications, Library, System and (hidden) private folders are being excluded by some process (not the user, and the exclusions do not appear on the Time Machine Preferences window). But all backups complete normally, and Verify Backups from the Time Machine menubar on network backups, and Verify Disk or Repair Disk via Disk Utility, don't show a problem (because what's there is intact; those processes can't tell that what's missing isn't supposed to be missing.)
In addition, we've seen a few threads where the problem is intermittent -- backups are unexpectedly large, sometimes often, sometimes less so, intermittently. In those cases, the folders are backed up, then skipped, then backed-up again, then skipped, etc. Since they appear to be "new" when backed-up again, they're backed-up in full, making backups of 15 GB or more.
See #D10 in Time Machine - Troubleshooting for an easy way to tell if you're affected, and a fix.
However, as we have no clue as to the cause yet, we need to be sure Apple knows about this, in detail, while it's happening, so I'm asking for some folks who are having these problems, and are fairly comfortable with the Finder at least, and following directions to do other things you may not be so familiar with, to help us collect information and contact Apple before running the fix in the link above (as that will destroy some of the evidence).
We're especially interested in folks who are covered by AppleCare, as you can call them for free and get some fairly quick attention. We can supply some "ammunition" for when you call them (since I haven't had this problem, and don't know how to reproduce it, I can't report it myself).
If you're having this problem and just want to run the fix, by all means do so (and check it periodically), but please don't post back with just a "me, too" post, unless you have something significant to add -- this thread may get long and those won't really help.
But if you're having a different problem, please do not respond to this thread. Start a new one, instead. If you're not sure how to do that, please go to the main Apple Support Communities page; some Tutorials are listed at the right. They'll show you how to get started.
