Sometimes Ruby is Weird

nil_what_are_you_doing_stahp.rb

class NilWhatAreYouDoingStahp
  attr_accessor :not_nil

  def initialize
    @not_nil = "I'm not nil! I'm a value!"
  end

  def wat
    if not_nil.nil?
      not_nil = "Ok, now I'm not nil."
    end
    not_nil
  end

  def omglol
    not_nil
  end
end

stahp = NilWhatAreYouDoingStahp.new
stahp.wat # => nil
stahp.omglol # => "I'm not nil! I'm a value!"

Spot on! Another fun way to present this puzzle is to say that adding five characters makes the code work as expected. That's almost a giveaway though.