Index: wp-includes/query.php
===================================================================
--- wp-includes/query.php	(revision 17497)
+++ wp-includes/query.php	(working copy)
@@ -1654,6 +1654,9 @@
 		}
 
 		foreach ( $GLOBALS['wp_taxonomies'] as $taxonomy => $t ) {
+			if ( 'post_tag' == $taxonomy )
+				continue;	// handled further down
+
 			if ( $t->query_var && !empty( $q[$t->query_var] ) ) {
 				$tax_query_defaults = array(
 					'taxonomy' => $taxonomy,