bug_test.rb

class BugTest1 < Hash
  
  # Any interface will do
  include Java::JavaLang::Runnable

  def run
    puts "Hello from BugTest1"
  end
end

class OrderHash < Hash
end

class BugTest2 < OrderHash
  
  # Any interface will do
  include Java::JavaLang::Runnable
  
  def run
    puts "Hello from BugTest2"
  end
end

# Works just fine
BugTest1.new

# ClassCastException
BugTest2.new

stacktrace.txt

jruby bug_test.rb 
org/jruby/RubyHash$i_method_0_1$RUBYFRAMEDINVOKER$initialize.gen:65535:in `call': java.lang.ClassCastException: BugTest2_1366331860 cannot be cast to org.jruby.RubyHash
        from RubyClass.java:540:in `finvoke'
        from RuntimeHelpers.java:450:in `invoke'
        from JavaInterfaceTemplate.java:275:in `call'
        from DynamicMethod.java:184:in `call'
        from DynamicMethod.java:180:in `call'
        from CachingCallSite.java:293:in `cacheAndCall'
        from CachingCallSite.java:112:in `call'
        from bug_test.rb:28:in `__file__'
        from bug_test.rb:-1:in `load'
        from Ruby.java:686:in `runScript'
        from Ruby.java:569:in `runNormally'
        from Ruby.java:412:in `runFromMain'
        from Main.java:304:in `run'
        from Main.java:144:in `run'
        from Main.java:113:in `main'