Example of how to log the request data from an invalid form submission

example.php

<?php

if ( ! function_exists('write_log')) {
   function write_log ( $log )  {
      if ( is_array( $log ) || is_object( $log ) ) {
         error_log( print_r( $log, true ) );
      } else {
         error_log( $log );
      }
   }
}

add_action('AHEE__EE_Form_Section_Proper__receive_form_submission__end', 'tw_ee_log_invalid_form', 10, 3);
function tw_ee_log_invalid_form( $req_data, $form, $validate) {
    if(! $form->is_valid() )  {
       write_log( $req_data);
    }
}