If you're running CouchDB 1.0.0 and 'delayed_commits' is set to true, the default setting, then you might be losing lots of data.
"Over the weekend of August 7th8th, 2010 we discovered and fixed a nasty bug in CouchDB 1.0.0," the CouchDB announcement says. "The problem was subtle (cancelling a timer, without deleting the reference to it) but the ramifications are not: once the bad code path is triggered, subsequent writes to the database are never committed. This means there is potential data-loss for users of 1.0.0."
This bug does not affect previous version of CouchDB.
There's also a hotfix available, and a permanent fix will be released this week. You can learn more about this bug and the fix here.