Based on this conversation in the driver's spec for pinning a transaction to a single mongos, I found that mongos returns NoSuchTransaction if the mongos receives a request for a transaction number different from the last transaction number it had heard but without 'startTransaction'. Perhaps this error message can be more clear? I had assumed the error would be something like "cannot start new transaction unless 'startTransaction' is specified."

There's a TODO in the code without a SERVER ticket, so I'm just filing this ticket to go back to it.