https://github.com/mongodb/mongo-ruby-driver/blob/master/lib/mongo/server/connection_pool/queue.rb#L99

http://ruby-doc.org/core-2.2.0/Array.html#method-c-new

Array#new/2 will populate the array with the given default value multiple times. This makes everything go to hell if you specify min_pool_size in a multi-threaded app, since the same connection instance can be checked out multiple times and then you get lovely things like cross-talk between messages.

The fix is here: https://github.com/cheald/mongo-ruby-driver/commit/2ae036667c04db60519ab4fa397b1add30628464

Though, I am curious, why is Ruby's Queue construct (which is threadsafe and already heavily battle-tested) not being used here?