This is a regression originally introduced in driver version 2.12. In RUBY-2241, we fixed the regression by modifying OP_MSG to validate that all documents are smaller than the maxBsonObjectSize before sending them to the server. However, we have not implemented the same behavior on OP_QUERY. This means that, when connected to pre-3.6 servers, the driver will validate that the total message size is less than the maxBsonObjectSize + overhead, but it will not check that individual documents fall below the maxBSonObjectSize. If the driver writes a document that falls between maxBsonObjectSize and maxBsonObjectSize + overhead, the server will error on that command, which is different behavior from driver version 2.11.