$ rbenv local 1.9.3-p484; ruby -v; ruby the_bug.rb
ruby 1.9.3p484 (2013-11-22 revision 43786) [x86_64-darwin13.0.0]
mooooo!
guarded mooooo!

$ rbenv local 2.0.0-p353; ruby -v; ruby the_bug.rb
ruby 2.0.0p353 (2013-11-22 revision 43784) [x86_64-darwin13.0.0]
mooooo!
the_bug.rb:20:in `<main>': undefined method `protected_moo' for #<Cow:0x007ffc9c0b63f0> (NoMethodError)

$ rbenv local 2.1.0; ruby -v; ruby the_bug.rb
ruby 2.1.0p0 (2013-12-25 revision 44422) [x86_64-darwin13.0]
mooooo!
the_bug.rb:20:in `<main>': undefined method `protected_moo' for #<Cow:0x007fc8021cdeb0> (NoMethodError)