Back in the day when all writes were unacknowledged, if a driver wanted to acknowledge the writes it had to issue a write + a getlasterror command. The Ruby driver contains an optimization for this use case allowing multiple messages to be sent in a single (high-level) operation.

Currently there is nothing in the driver making use of this functionality - the driver always sends one message at a time. We have no test coverage for sending multiple messages, and the code looks like it's wouldn't correctly handle this case. To avoid changing the API the driver will continue to accept messages as an array; however, this ticket is for raising an exception when the application asks the driver to dispatch multiple messages on the assumption that such a request wouldn't be correctly handled anyway.