>> Hash.new { raise }
=> {}
>> h = Hash.new { raise }
=> {}
>> h[:a] = "yay"
=> "yay"
>> h[:a]
=> "yay"
>> h[:b]
RuntimeError: 
  from (irb):2
  from (irb):5:in `call'
  from (irb):5:in `default'
  from (irb):5:in `[]'
  from (irb):5