The following works on Ruby 1.6.5:

  class Foo
    class << self
      def foo
        @@x = Hash.new
        @@x[0] = 1
      end
    end
  end

  Foo.foo

but on Ruby 1.6.7 I get:

  -:5:in `foo': uninitialized class variable @@x in Foo (NameError)
          from -:10