I ran into this problem, that might be relevant for other users as well:
1) Create document with couch-client, receiving _rev 1-X
2) Change document NOT with couch-client, i.e. by using Futon, resulting in _rev 2-Y
3) Call save() again from couch-client. => Will NOT cause error in callback, but faulty doc (_rev undefined).
The actual CouchDB response to the bulk_docs POST contains the appropriate error (conflict), which is not handled by couch-client however.
I'm using CouchDB 1.0.2, couch-client 0.0.4 and node.js 0.4.7
Thanks for your help!