Ticket #23329: 23329.2.patch

wp-includes/query.php

@@ -720 +720 @@
  * @return bool
  */
 function is_main_query() {
+        if ( 'pre_get_posts' == current_filter() )
+                _doing_it_wrong( __FUNCTION__, __( 'is_main_query() should not be used within the pre_get_posts action as it may not reflect the current query object.' ), '3.6' );
+
         global $wp_query;
         return $wp_query->is_main_query();
 }