Form Validation Callback Executing First

I'm very late to the party with this but just for future reference:

It's happening because at least one of your callbacks is loading another module. This is basically causing the validation callback to get mixed up because it's checking the 'called' module for the method and it cannot find it.

The solution is to have your validation callbacks contained with the module from which they are called. This is less than perfect but at least it'll get you on the road again.