http://www.jstorimer.com/blogs/workingwithcode/8085491-nobody-understands-the-gil Appending to arrays is not thread-safe Very few things are implicitly thread-safe in Ruby. For example, appending to arrays:

threading.rb

array = []

5.times.map do
  Thread.new do
    1000.times do
      array << nil
    end
  end
end.each(&:join)

puts array.size