Index: wp-includes/post.php
===================================================================
--- wp-includes/post.php	(revision 17341)
+++ wp-includes/post.php	(working copy)
@@ -2790,8 +2790,11 @@
 		// namespace than posts so page slugs are allowed to overlap post slugs.
 		$check_sql = "SELECT post_name FROM $wpdb->posts WHERE post_name = %s AND post_type IN ( '" . implode( "', '", esc_sql( $hierarchical_post_types ) ) . "' ) AND ID != %d AND post_parent = %d LIMIT 1";
 		$post_name_check = $wpdb->get_var( $wpdb->prepare( $check_sql, $slug, $post_ID, $post_parent ) );
+		
+		// top-level pages cannot have the same slug as the tag/category bases
+		$category_tag_base_check = ( 0 == $post_parent ) ? in_array( $slug, array( get_option( 'category_base' ), get_option( 'tag_base' ) ) ) : false;
 
-		if ( $post_name_check || in_array( $slug, $feeds ) || preg_match( "@^($wp_rewrite->pagination_base)?\d+$@", $slug )  || apply_filters( 'wp_unique_post_slug_is_bad_hierarchical_slug', false, $slug, $post_type, $post_parent ) ) {
+		if ( $post_name_check || $category_tag_base_check || in_array( $slug, $feeds ) || preg_match( "@^($wp_rewrite->pagination_base)?\d+$@", $slug )  || apply_filters( 'wp_unique_post_slug_is_bad_hierarchical_slug', false, $slug, $post_type, $post_parent ) ) {
 			$suffix = 2;
 			do {
 				$alt_post_name = substr( $slug, 0, 200 - ( strlen( $suffix ) + 1 ) ) . "-$suffix";