Here's an example you can play with from José Valim's talk on concurrency. Set num_threads to anything above 1. Use jruby for maximum thread conflict chance.

race_condition_example.rb

# run me with jruby so that race conditions happen more frequently

# run with num_threads = 1 and then change num_threads to anything above 1.
# notice the difference?  :D
num_threads = 1

require 'active_support'

class Counter
  cattr_accessor :i
  self.i = 0

  def self.count_up
    self.i = self.i + 1
  end
end

count_to = 1000
iterations = 0

num_threads.times.map do
  Thread.new {
    while iterations < count_to
      Counter.i = Counter.i + 1
      iterations += 1
    end
  }
end.each(&:join)

puts Counter.i
raise "Race condition foo!" if Counter.i != count_to