Title is a mouthful, but the basic issue is that if a module is included, then modified by including other modules into itself, then included again, the new includes don't get spliced into the target hierarchy correctly.

The following example shows that the order of modules is different from MRI:

Output:

[--dev]
~/projects/jruby $ rvm ruby-2.1 do ruby blah.rb
in Z
in A
in Q

[--dev]
~/projects/jruby $ jruby blah.rb
in A
in Z
in Q

Script:

module X
end

class Q
  def foo
    puts 'in Q'
  end
end

class Y < Q
  include X
end

module A
  def foo
    puts 'in A'
    super
  end
end

module X
  include A
end

module Z
  def foo
    puts 'in Z'
    super
  end
end

class Y
  include Z
end

class Y
  include X
end

Y.new.foo

This is the likely cause of Compass/compass-rails#179.