http://groups.google.com/group/mongodb-user/browse_thread/thread/5ea1525106b6805e

"This specific issue turned out to be caused by thread closing to
early, shutting down the mongoconnection but keeping the rabbitMQ
connection alive. So when a message arrived it tried to access mongo
through a closed connection."

I'm guessing that the sequence goes something like this:

1. Thread A checks out a socket.
2. Thread B calls Connection#close (or perhaps there's some kind of socket error that closes connections)
3. Thread A attempts to write on socket, fails because the socket has been close, and itself tries to close the socket.

Probably need to synchronize access to @sockets in Pool class.