> had never been reported before, despite being largely the same for 9 years

I'll admit to encountering weird "er, why is that randomly nil" errors quite regularly with JRuby when load testing webservers, which, er, I've never reported. It's never been obvious where the fault was, really. And it always seemed unlikely to be your fault, tsk ;)

> It's better to avoid using closure state if you know it's going to be called across threads

Yeah. In this case it was a hash of name -> lambda pairs which boiled down to variations on:

    ->(obj) do
      case obj.bla
        when /foo(.*)/ then "FOO_#{$1.upcase}"
        when /bla(.*)/ then $1
      end
    end

Called in lots of tight loops from a pool of worker threads. I refactored it into something neater, but it still should have worked fine :)