In 2.12.0 the new bulk write operations, when executing against a pre 2.6 server, treat a getlasterror response with wnote, jnote, or wtimeout as exceptions. But it turns out that pre-2.6 servers use wnote and jnote for things other than errors. For example, when issuing a remove with a query that matches no documents, the server may respond with a wnote:

{ ... "wnote" : "no write has been done on this connection", "err" : null, "ok" : 1.0 ... }

Given that, the driver should remove special handling of jnote and wnote.

Impact of this bug is low, as it only affects the new bulk write operations, and then only when running against pre-2.6 servers.