Ran into this in our error grabber. Not sure if it's useful or not, but it smells like the same connection (/socket) was being concurrently used by two threads.

org/jruby/RubyIO.java:3673:in select
from gems/mongo-1.10.1-java/lib/mongo/connection/socket/tcp_socket.rb:70:in read
from gems/mongo-1.10.1-java/lib/mongo/networking.rb:344:in receive_data
from gems/mongo-1.10.1-java/lib/mongo/networking.rb:329:in receive_message_on_socket
from gems/mongo-1.10.1-java/lib/mongo/networking.rb:189:in receive_header
from gems/mongo-1.10.1-java/lib/mongo/networking.rb:180:in receive
from gems/mongo-1.10.1-java/lib/mongo/networking.rb:144:in receive_message
from gems/mongo-1.10.1-java/lib/mongo/cursor.rb:543:in send_initial_query
from gems/mongo-1.10.1-java/lib/mongo/cursor.rb:540:in send_initial_query
from gems/mongo-1.10.1-java/lib/mongo/functional/logging.rb:55:in instrument
from gems/mongo-1.10.1-java/lib/mongo/functional/logging.rb:20:in instrument
from gems/mongo-1.10.1-java/lib/mongo/functional/logging.rb:54:in instrument
from gems/mongo-1.10.1-java/lib/mongo/cursor.rb:539:in send_initial_query
from gems/mongo-1.10.1-java/lib/mongo/cursor.rb:524:in refresh
from gems/mongo-1.10.1-java/lib/mongo/cursor.rb:136:in next
from gems/mongo-1.10.1-java/lib/mongo/db.rb:556:in command
from gems/mongo-1.10.1-java/lib/mongo/connection/pool.rb:151:in ping
from gems/mongo-1.10.1-java/lib/mongo/connection/pool.rb:131:in refresh_ping_time
from org/jruby/RubyFixnum.java:275:in times
from gems/mongo-1.10.1-java/lib/mongo/connection/pool.rb:129:in refresh_ping_time
from gems/mongo-1.10.1-java/lib/mongo/connection/pool.rb:118:in ping_time
from org/jruby/ext/thread/Mutex.java:149:in synchronize
from gems/mongo-1.10.1-java/lib/mongo/connection/pool.rb:116:in ping_time
from gems/mongo-1.10.1-java/lib/mongo/functional/read_preference.rb:167:in select_near_pool
from org/jruby/RubyArray.java:1613:in each
from org/jruby/RubyEnumerable.java:1215:in min_by
from gems/mongo-1.10.1-java/lib/mongo/functional/read_preference.rb:167:in select_near_pool
from gems/mongo-1.10.1-java/lib/mongo/functional/read_preference.rb:162:in select_secondary_pool
from gems/mongo-1.10.1-java/lib/mongo/functional/read_preference.rb:138:in select_pool
from gems/mongo-1.10.1-java/lib/mongo/functional/read_preference.rb:115:in read_pool
from gems/mongo-1.10.1-java/lib/mongo/mongo_replica_set_client.rb:359:in checkout_reader
from gems/mongo-1.10.1-java/lib/mongo/mongo_replica_set_client.rb:343:in checkout
from gems/mongo-1.10.1-java/lib/mongo/mongo_replica_set_client.rb:358:in checkout_reader
from gems/mongo-1.10.1-java/lib/mongo/cursor.rb:615:in checkout_socket_from_connection
from gems/mongo-1.10.1-java/lib/mongo/cursor.rb:542:in send_initial_query
from gems/mongo-1.10.1-java/lib/mongo/cursor.rb:540:in send_initial_query
from gems/mongo-1.10.1-java/lib/mongo/functional/logging.rb:55:in instrument
from gems/mongo-1.10.1-java/lib/mongo/functional/logging.rb:20:in instrument
from gems/mongo-1.10.1-java/lib/mongo/functional/logging.rb:54:in instrument
from gems/mongo-1.10.1-java/lib/mongo/cursor.rb:539:in send_initial_query
from gems/mongo-1.10.1-java/lib/mongo/cursor.rb:524:in refresh
from gems/mongo-1.10.1-java/lib/mongo/cursor.rb:513:in num_remaining
from gems/mongo-1.10.1-java/lib/mongo/cursor.rb:187:in has_next?