Find where a variable is overwritten with a different value

overwritten.rb

class Config
  attr_accessor :thing

  def thing=(val)
    raise ArgumentError if defined?(@thing)
    @thing = val
  end
end

config = Config.new
config.thing = { "foo" => "bar" }
config.thing = nil

# overwritten.rb:5:in `thing=': ArgumentError (ArgumentError)
#	from overwritten.rb:12:in `<main>'

From the Missing Pieces Wish List on http://www.schneems.com/2016/01/25/ruby-debugging-magic-cheat-sheet.html