Incidentally, this happened to me yesterday. I did a backup and noticed two things, the screen didn't shut off as it normally does during syncing and the meter stated I was over capacity or something to that affect even though I knew I had at least 15GBs of free space.
Didn't really think anything of it till 2hrs later when none of my 3rd party apps loaded. All the native apps opened without a problem. Did a restore and so far so good as if nothing happened.