It looks like ever since this commit we started unconditionally dropping the target collection when we're finished with $out mode "replaceCollection". Before this, we would only drop the temp namespace if the command didn't fully succeed since we reset _tempNS after finishing the inserts. Because all of this is happening within the DESTRUCTOR_GUARD, the exception is swallowed and there is no error reported back to the client. So there is no actual impact of this change, but we could probably gain back a little performance and sanity by avoiding dropping a non-existent collection at the end of every $out.