BSON::OrderedHash defines

      def clone
        Marshal::load(Marshal.dump(self))
      end

on ruby 1.8:

    # We only need the body of this class if the RUBY_VERSION is before 1.9
    if RUBY_VERSION < '1.9'

However, #clone is, in general, specified to be a shallow clone, and in fact Hash#clone is shallow. The net result is that BSON::OrderedHash#clone has different semantics in 1.8 and 1.9.