Form API multiple #validate records I have an interesting question.
There is a custom module that defines form -> form validate and form submit

I am not allowed to touch this module, still I want to execute another custom action on submit (OR validate).

I have added 
$form['#validate'][]='_custom_validator';  in my hook_form_alter
So the $form array has two functions that validate.
And that seems to work, the problem is I have to "copy-paste" the validation code from the other module (the code inside hook_form_validate).(To prevent any incorrect form validation). 
This is considered as a bad design pattern.

Question:
Is there somewhere a property that defines the order of the validations ?

Cheers
You can use array_unshift to make sure your validation is called first:

array_unshift($form['#validate'], '_custom_validator');