When defining new on a class singleton that gets aliased to another name, super inside the class dispatches to the aliased, not the original name. MRI and RBX both behave different here and dispatch to superclasses original named method.

crash.rb

class Foo

  def self.new
    super
  end

  class << self
    alias_method :bar, :new
  end

end

p Foo.bar

jruby:

jruby 1.7.4 (1.9.3p392) 2013-05-16 2390d3b on OpenJDK 64-Bit Server VM 1.7.0_40-b31 +indy [linux-am

mbj@mbj ~/devel/crash (master*) % jruby crash.rb
NoMethodError: super: no superclass method `bar'
     new at crash.rb:4
  (root) at crash.rb:13

cruby:

ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-linux]

mbj@mbj ~/devel/crash (master*) % ruby crash.rb
#<Foo:0x007f0f1d4fc580>

I reduced this bug from around 20kloc, and hope you can fix it in the next minor release. I can trigger this very easily from my production code. Using 1.7.2 now where this bug does not exist.