Array_outbound.rb

# (from: http://melborne.github.io/2013/08/30/monkey-patching-for-prudent-rubyists/)
class Array
  alias :get_at :[]
  private :get_at

  alias :set_at :[]=
  private :set_at

  def in_bound?(idx)
    0 <= idx && idx < self.size
  end

  # NOTE: Overwrite  
  def [](idx)
    if self.in_bound?(idx)
      get_at(idx)
    else
      raise IndexError.new("index #{idx} outside of array bounds: 0...#{self.size}")
    end
  end

  # NOTE: Overwrite  
  def []=(idx, val)
    if self.in_bound?(idx)
      set_at(idx, val)
    else
      raise IndexError.new("index #{idx} outside of array bounds: 0...#{self.size}")
    end
  end
end

if __FILE__ == $0
  begin
    p([1, 2, 3][10])
  rescue
    puts("Should be printed!")
  end

  arr  = [1, 2, 3]
  arr[0] = 10
  p(arr)
end

It may be useful to debug index problem.