Steps to reproduce:

• Start with 3-node replica set, running on 127.0.0.1:8001, 127.0.0.1:8002, 127.0.0.1:8003. Have 127.0.0.1:8003 be the master.
• Run m = Mongo::ReplSetConnection.new(['127.0.0.1', 8001], ['127.0.0.1', 8002], ['127.0.0.1', 8003]). Run a query.
• From :8003, do rs.remove('127.0.0.1:8001')
• From :8003, do rs.stepDown() to force the driver to reconnect.
• Try running another query. It will fail with "Mongo::ConnectionFailure: Failed to connect to primary node." because :8001 is still up and thinks it's a replica set (in my testing, it's now in STARTUP mode).