Possible bug: ext3 misreporting filesystem usage

I've been getting a possible bug after running my system a few weeks. The ext3 partition's usage is being misreported. Right now, df -h says ive got no space left, but according to du /, I'm only using 17 gigs of my 40 gig drive. Restarting fixes the problem, so I'm thinking it might be some mis-handled variable in memory, not something on the disc itself? And, yes, I do know that du is right, not df, because I keep good track of my disc usage. This is pretty serious, it killed a 40+ hour process that i'll have to start over again from the beginning!