Index: wp-includes/taxonomy.php
===================================================================
--- wp-includes/taxonomy.php	(revision 18473)
+++ wp-includes/taxonomy.php	(working copy)
@@ -311,6 +311,9 @@
 					);
 	$args = wp_parse_args($args, $defaults);
 
+	if ( false === $args['_builtin'] && ! empty( $wp ) && in_array( $taxonomy, array_merge( $wp->public_query_vars, $wp->private_query_vars ) ) )
+		_doing_it_wrong( __FUNCTION__, __( 'You should not use a reserved term when registering a taxonomy.' ), '3.3' );
+	
 	if ( false !== $args['query_var'] && !empty($wp) ) {
 		if ( true === $args['query_var'] )
 			$args['query_var'] = $taxonomy;