I performed the following sequence of operations:

• Connected to a 3-node replica set using 1.5.1:

>> m = Mongo::ReplSetConnection.new(['127.0.0.1', 6001], ['127.0.0.1', 6002], ['127.0.0.1', 6003]); m.primary
=> ["qa-dev1.stripe.com", 6002]

• Did an rs.stepDown() on the master
• On the next request, as expected, an error was raised:

>> m['test']['tar'].insert({}, :safe => true)
Mongo::ConnectionFailure: Operation failed with the following exception: Connection reset by peer
from /usr/lib/ruby/gems/1.8/gems/mongo-1.5.1/lib/../lib/mongo/networking.rb:312:in `receive_message_on_socket'
from /usr/lib/ruby/gems/1.8/gems/mongo-1.5.1/lib/../lib/mongo/networking.rb:169:in `receive_header'

from /usr/lib/ruby/gems/1.8/gems/mongo-1.5.1/lib/../lib/mongo/collection.rb:944:in `insert_documents'
...

• On the next request, it appears the connection is trying to write to the wrong node:

>> m['test']['tar'].insert({}, :safe => true)
Mongo::OperationFailure: 10058: not master
from /usr/lib/ruby/gems/1.8/gems/mongo-1.5.1/lib/../lib/mongo/networking.rb:82:in `send_message_with_safe_check'
from /usr/lib/ruby/gems/1.8/gems/mongo-1.5.1/lib/../lib/mongo/collection.rb:946:in `insert_documents'

• Indeed, the primary hasn't updated:

>> m.primary
=> ["qa-dev1.stripe.com", 6002]