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If you're using TimeMachine, and trusting that all your important data has been safely backed up in multiple places, you could well be wrong.

Don't get me wrong -- automatic backups are great. Apple's TimeMachine works pretty well. Except when it doesn't.

TimeMachine has bugs. It is quite capable of forgetting to backup directories that have changed, and does nothing to verify that your backed up data is actually complete. I've reported these bugs to Apple, who closed them as duplicates.

If you've ever seen the "Verify Backups" option, note that this merely verifies that the remote disk image isn't corrupt -- it doesn't actually check that the backup set contains what it should contain.

All is not lost, though -- you can issue "tmutil compare" from the command line to get TimeMachine to compare your mounted backup with what should be backed up -- if you're using TimeMachine I *seriously* recommend you run this command right now. It will churn for awhile, and will output a list of files that differ between your backup and your live system. A little bit of scripting will turn the output of "tmutil compare" into a "touch script" that will help to encourage TimeMachine to backup all the files it has missed. The fact that TimeMachine doesn't do this automatically every month or so is pretty sad.

To be fair, I haven't seen TimeMachine drop files since I initially discovered this problem, but the directory it did decide to ignore was a seriously important directory, and I've been running "tmutil compare" monthly since then. If I'd had a drive failure and recovered my system from backup, I would have been very very pissed at what was not included.