this is the backtrace:

<internal:prelude>:8:in `lock'
<internal:prelude>:8:in `synchronize'
mongo (1.3.0) lib/mongo/connection.rb:808:in `get_request_id'
mongo (1.3.0) lib/mongo/connection.rb:789:in `add_message_headers'
mongo (1.3.0) lib/mongo/connection.rb:446:in `receive_message'
mongo (1.3.0) lib/mongo/cursor.rb:392:in `block in send_initial_query'
mongo (1.3.0) lib/mongo/connection.rb:585:in `instrument'
mongo (1.3.0) lib/mongo/cursor.rb:391:in `send_initial_query'
mongo (1.3.0) lib/mongo/cursor.rb:356:in `refresh'
mongo (1.3.0) lib/mongo/cursor.rb:75:in `next_document'
mongo (1.3.0) lib/mongo/collection.rb:242:in `find_one'
/usr/local/rvm/gems/ruby-1.9.2-p180/bundler/gems/mongoid-b760004d5bc4/lib/mongoid/collections/master.rb:19:in `block (3 levels) in <class:Master>'
/usr/local/rvm/gems/ruby-1.9.2-p180/bundler/gems/mongoid-b760004d5bc4/lib/mongoid/collections/retry.rb:29:in `retry_on_connection_failure'
/usr/local/rvm/gems/ruby-1.9.2-p180/bundler/gems/mongoid-b760004d5bc4/lib/mongoid/collections/master.rb:18:in `block (2 levels) in <class:Master>'
/usr/local/rvm/gems/ruby-1.9.2-p180/bundler/gems/mongoid-b760004d5bc4/lib/mongoid/collection.rb:71:in `find_one'
/usr/local/rvm/gems/ruby-1.9.2-p180/bundler/gems/mongoid-b760004d5bc4/lib/mongoid/contexts/mongo.rb:248:in `one'
/usr/local/rvm/gems/ruby-1.9.2-p180/bundler/gems/mongoid-b760004d5bc4/lib/mongoid/criteria.rb:48:in `first'