Your block database is probably corrupted. Shut down Bitcoin. Go to the
data directory and move the
chainstate and
blocks directories somewhere else. (Keep them for a while in case the devs are interested in looking at them.) Then start Bitcoin, and it'll resync. You could also try deleting just the
chainstate directory for a faster resync, but this might not actually fix it.
FYI: My block database was also corrupted recently with unmodified (but self-built) 0.8.5 on Gentoo. I wonder if a recent block/tx triggered this on certain systems.