Demonstration of nil backtraces in Ruby. nil backtraces show up sometimes in MRI. In JRuby-1.7.3, NullPointerExceptions show up for the backtracing thread.

thread-bt-demo.rb

#!/usr/bin/env ruby

Thread.new do
  loop do
    Thread.list.each do |t|
      puts "Got a nil backtrace for thread #{t}" if t.backtrace.nil?
    end
  end
end

loop { Thread.new { sleep 0.001 }.join }