Apparently, there's a bug in the latest stable kernels affecting kernel 3.4, 3.5, and 3.6. If you use ext4, then there's a possibility of triggering this under certain circumstances as reported on Phoronix.
And just when Ubuntu 12.10 got released not too long ago, too. Hopefully, a fix will be available soon-ish.
Update: Phoronix reports a patch to fix this is available. We'll likely see the patch cherry picked for the Ubuntu kernel soon.