Given code:

foo2 = 'dummy2'
result =  begin
    foo = 'dummy'
    foo
  ensure
    foo = foo2
  end
puts result, result.class

It works differently under MRI 2.2.1 and JRuby 9.0.1.0:

MRI 2.2.1:

dummy

JRuby 9.0.1.0:

dummy2

Most cases this won't be problem, but when you want to execute a block of code with a variable temporarily set to another values using the begin....ensure technique, you will get unwanted result. For example, tilt will not be able process erb files with JRuby due to this.