Steps to reproduce:

1. Deploy a single node replica set with MMAPv1:

mongod --port 27017 --fork --dbpath ./data/ --logappend --storageEngine mmapv1 --logpath "./mongodb.log" --replSet rubyTest

2. Initiate the replica set

mongo --port 27017
use admin
rs.initiate()

3. Connect to the deployment with a connection string containing arguments retryWrites=false&retryReads=false. See test3.rb attached.

4. Attempting to results in "Transaction numbers are only allowed on storage engines that support document-level locking" error. See error.txt attached.

I was able to reproduce this on 4.0 as well.