Wierd to_a issue

When I go to save a model, I get tons of these:

associations/association_proxy.rb:214: warning: default `to_a’ will be

It looks like the warning is coming from method_missing!

Even stranger: the model saves correctly in the development
environment, but under certain conditions it does not save in the
production environment – even if operating with an identical

Any ideas of what could be going on here? I can’t find anything about
this and am getting frustrated. Any tips on how to trace where the
warnings are coming from would be helpful.