Can somebody take a look at the discussion here:
https://github.com/rails/rails/pull/12607
(suggested fix at the bottom)
tl;dr: there's an obscure race condition which only happens when two threads dispatch into method_missing on instances of the same ActiveRecord model. The net result is that one of them successfully generates + calls the requested method while the other fails with NoMethodError. 
The fix is (as far as I can tell) pretty straightforward, so this would be great to get into 4.0.1.
--Matt Jones
