gistfile1.rb

module A
  class B
    def shit(x)
      puts "I AM THE ORIGINAL #{x}"
      x + 1
    end

    alias_method :oops, :shit
  end
end

a = A::B.new.shit(1)
b = A::B.new.oops(1)

module A
  class B
    alias_method :_old_shit, :shit
    def shit(x)
      puts "I AM THE NEW SHIT #{x}"
      x + 10
    end

#    alias_method :oops, :shit
  end
end

c = A::B.new.shit(1)
d = A::B.new.oops(1)

puts "RESULTS: #{a} - #{b} - #{c} - #{d}"

With L23 commented out, it prints
RESULTS: 2 - 2 - 11 - 2

With L23 not commented out, it prints
RESULTS: 2 - 2 - 11 - 11