Wierd to_a issue...

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

/usr/lib/ruby/gems/1.8/gems/activerecord-2.2.2/lib/active_record/ associations/association_proxy.rb:214: warning: default `to_a' will be obsolete

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 database.

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.