In MRI, Fatal is not defined.

But in JRuby, it is.

Code that expects to be able to create it's own module Fatal or otherwise will fail.

In MRI, fatal is not exposed to user. But you can access it by using ObjectSpace to find it.

I suggest that initially, you remove class Fatal from global namespace. Then, perhaps with agreement from MRI, we can make something like FatalError, perhaps defined under RubyVM or something else.