gistfile1.rb

class Puzzle
  attr_accessor :field
  
  def initialize
    self.field = true
  end
    
  def run
    puts field.inspect            # => true
    if false
      field = "NEVER SET"
    end    
    puts field.inspect            # => ?
  end
end

Puzzle.new.run

Hint:

1.8.7-p330 :001 > if false ; x = 5 ; end
 => nil 
1.8.7-p330 :002 > x
 => nil 
1.8.7-p330 :003 > y
NameError: undefined local variable or method `y' for #<Object:0x3794c>
    from (irb):3