Once it happens it keeps poping up and I have to kill NetBeans instance.

1) Create new Ruby project
2) I've copy pasted this content into the main.rb

  class My
    @a = 10
    def initialize; @x = 10; end
    def fib(n)
      @fib_cache ||= [1, 1]
      @fib_cache[n] ||= fib(n-1) + fib(n-2)
    end
  end

  my = My.new
  puts my.fib(20)
  sleep 0.1

3) Click to another opened file and back
4) Exception start to poping up infinitely

Attaching exception.