So, go into Disk Utility and verify your drives, especially the one you use for Time Machine.
I had my drive hooked up to and Airport Extreme Base Station and Time Machine saving to that. At night, I'd go to the Apple menu and put my computer to sleep. I'd notice a backup was occurring and worry a bit but figure Apple would have thought of that. But it did seem to sleep awfully fast considering it should have been stopping the network backup first.
When I would wake the computer the next day, I'd get the message saying the backup had failed. This happened a couple of days in a row before I noticed that fseventsd was going crazy.
I found a web page saying fseventsd doesn't deal with corrupted disks well, so I detached the drive from the AEBS and hooked it directly to the computer. Sure enough the drive had some errors. The sparse bundle that networked Time Machine creates was so bad it was unverifiable and unfixable. Something about a link being bad somewhere. I bet when I put the computer to sleep it just stopped the backup abruptly.
So I had to start all over. And I lost all my backup history. Luckily for me, that's not a huge deal, but it could have been (or could be, if I figure out I'm missing something.)
Moral of the story, don't put your computer to sleep if Time Machine is currently running. I don't know if this is a problem with directly attached drives as well, but I wouldn't risk it.
