In MRI 1.9 when adding the optional flag to const_get it doesn't just prevent lookup in superclasses but also at the toplevel.
Basically Enumerable.const_get(:Array, false) raises a NameError in MRI.
In JRuby this is not the case.
I have created two RubySpec test cases that checks and expects the MRI behavior and therefore fails with JRuby. https://github.com/rubyspec/rubyspec/pull/86
It is also worth mentioning that constantize in activesupport expects and builds on the MRI behavior, causing subtle problems.