If this line throws, e.g. due to the object we're trying to construct being too large, this leaves the BSONObjBuilder in a state where "_doneCalled" is true, which can later lead to this invariant being hit unexpectedly. We should either make this function exception-safe and reset the state of the BSONObjBuilder to how it was before calling _done() in this line, if possible, and if not, we should update callers to be prepared for the possibility of this throwing.