Whenever the d.o DB is unavailable (not too uncommon a situation) and people do CVS operations, the data between CVS and d.o gets out of sync. Some examples:

#343796: CVS commits durring db1 outage not showing up on project pages
#343820: Drupal CVS tag won't show up as a possible release on project page
...

Over at #343820 greggles suggested preventing commits (and I'd add tags) whenever the DB is unavailable. It won't be completely fool-proof, since there will be race conditions between when we prevent/allow the CVS operation and when we actually need to update the data (since for commits its a totally different CVS hook that enforces the access and the one that records data about successful operations), but it'd be a start.