Index: wp-includes/default-filters.php
===================================================================
--- wp-includes/default-filters.php	(revision 15799)
+++ wp-includes/default-filters.php	(working copy)
@@ -117,6 +117,10 @@
 	add_filter( $filter, 'convert_chars' );
 }
 
+// Pre save hierarchy
+add_filter( 'wp_insert_post_parent', 'wp_check_post_hierarchy_for_loops', 10, 2 );
+add_filter( 'wp_update_term_parent', 'wp_check_term_hierarchy_for_loops', 10, 3 );
+
 // Display filters
 add_filter( 'the_title', 'wptexturize'   );
 add_filter( 'the_title', 'convert_chars' );
Index: wp-includes/taxonomy.php
===================================================================
--- wp-includes/taxonomy.php	(revision 15799)
+++ wp-includes/taxonomy.php	(working copy)
@@ -2039,6 +2039,9 @@
 		}
 	}
 
+	// Check $parent to see if it will cause a hierarchy loop
+	$parent = apply_filters( 'wp_update_term_parent', $parent, $term_id, $taxonomy, compact( array_keys( $args ) ), $args );
+
 	// Check for duplicate slug
 	$id = $wpdb->get_var( $wpdb->prepare( "SELECT term_id FROM $wpdb->terms WHERE slug = %s", $slug ) );
 	if ( $id && ($id != $term_id) ) {
@@ -2879,3 +2882,62 @@
 
 	return $tags_to_edit;
 }
+
+/**
+ * Returns the term's parent's term_ID
+ *
+ * @since 3.1
+ *
+ * @param int $term_id
+ * @param string $taxonomy
+ *
+ * @return int|bool false on error
+ */
+function wp_get_term_taxonomy_parent_id( $term_id, $taxonomy ) {
+	$term = get_term( $term_id, $taxonomy );
+	if ( !$term || is_wp_error( $term ) )
+		return false;
+	return (int) $term->parent;
+}
+
+/**
+ * Checks the given subset of the term hierarchy for hierarchy loops.
+ * Prevents loops from forming and breaks those that it finds.
+ *
+ * Attached to the wp_update_term_parent filter.
+ *
+ * @since 3.1
+ * @uses wp_find_hierarchy_loop()
+ *
+ * @param int $parent term_id of the parent for the term we're checking.
+ * @param int $term_id The term we're checking.
+ * @param string $taxonomy The taxonomy of the term we're checking.
+ *
+ * @return int The new parent for the term.
+ */
+function wp_check_term_hierarchy_for_loops( $parent, $term_id, $taxonomy ) {
+	// Nothing fancy here - bail
+	if ( !$parent )
+		return 0;
+
+	// Can't be its own parent
+	if ( $parent == $term_id )
+		return 0;
+
+	echo "larger loops\n";
+
+	// Now look for larger loops
+
+	if ( !$loop = wp_find_hierarchy_loop( 'wp_get_term_taxonomy_parent_id', $term_id, $parent, array( $taxonomy ) ) )
+		return $parent; // No loop
+
+	// Setting $parent to the given value causes a loop
+	if ( isset( $loop[$term_id] ) )
+		return 0;
+
+	// There's a loop, but it doesn't contain $term_id.  Break the loop.
+	foreach ( array_keys( $loop ) as $loop_member )
+		wp_update_term( $loop_member, $taxonomy, array( 'parent' => 0 ) );
+
+	return $parent;
+}
Index: wp-includes/post.php
===================================================================
--- wp-includes/post.php	(revision 15799)
+++ wp-includes/post.php	(working copy)
@@ -2320,17 +2320,8 @@
 	else
 		$post_parent = 0;
 
-	if ( !empty($post_ID) ) {
-		if ( $post_parent == $post_ID ) {
-			// Post can't be its own parent
-			$post_parent = 0;
-		} elseif ( !empty($post_parent) ) {
-			$parent_post = get_post($post_parent);
-			// Check for circular dependency
-			if ( isset( $parent_post->post_parent ) && $parent_post->post_parent == $post_ID )
-				$post_parent = 0;
-		}
-	}
+	// Check the post_parent to see if it will cause a hierarchy loop
+	$post_parent = apply_filters( 'wp_insert_post_parent', $post_parent, $post_ID, compact( array_keys( $postarr ) ), $postarr );
 
 	if ( isset($menu_order) )
 		$menu_order = (int) $menu_order;
@@ -4787,8 +4778,66 @@
 	}
 }
 
+/**
+ * Returns the post's parent's post_ID
+ *
+ * @since 3.1
+ *
+ * @param int $post_id
+ *
+ * @return int|bool false on error
+ */
+function wp_get_post_parent_id( $post_ID ) {
+	$post = get_post( $post_ID );
+	if ( !$post || is_wp_error( $post ) )
+		return false;
+	return (int) $post->post_parent;
+}
 
 /**
+ * Checks the given subset of the post hierarchy for hierarchy loops.
+ * Prevents loops from forming and breaks those that it finds.
+ *
+ * Attached to the wp_insert_post_parent filter.
+ *
+ * @since 3.1
+ * @uses wp_find_hierarchy_loop()
+ *
+ * @param int $post_parent ID of the parent for the post we're checking.
+ * @parem int $post_ID ID of the post we're checking.
+ *
+ * @return int The new post_parent for the post.
+ */
+function wp_check_post_hierarchy_for_loops( $post_parent, $post_ID ) {
+	// Nothing fancy here - bail
+	if ( !$post_parent )
+		return 0;
+
+	// New post can't cause a loop
+	if ( empty( $post_ID ) )
+		return $post_parent;
+
+	// Can't be its own parent
+	if ( $post_parent == $post_ID )
+		return 0;
+
+	// Now look for larger loops
+
+	if ( !$loop = wp_find_hierarchy_loop( 'wp_get_post_parent_id', $post_ID, $post_parent ) )
+		return $post_parent; // No loop
+
+	// Setting $post_parent to the given value causes a loop
+	if ( isset( $loop[$post_ID] ) )
+		return 0;
+
+	// There's a loop, but it doesn't contain $post_ID.  Break the loop.
+	foreach ( array_keys( $loop ) as $loop_member )
+		wp_update_post( array( 'ID' => $loop_member, 'post_parent' => 0 ) );
+
+	return $post_parent;
+}
+
+/**
  * Default post information to use when populating the "Write Post" form.
  *
  * @since unknown
Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 15799)
+++ wp-includes/functions.php	(working copy)
@@ -4338,4 +4338,74 @@
 	}
 }
 
-?>
+/**
+ * Finds hierarchy loops using a callback function that maps objects to parents.
+ *
+ * @since 3.1
+ *
+ * @param callback $callback function that accepts ( ID, callback_arg, ... ) and outputs parent_ID
+ * @param $start The ID to start the loop check at
+ * @param $start_parent the parent_ID of $start to use instead of calling $callback( $start ).  Use null to always use $callback
+ * @param array $override an array of ( ID => parent_ID, ... ) to use instead of $callback
+ * @param array $callback_arg optional additional arguments to send to $callback
+ *
+ * @internal
+ *
+ * @return array IDs of all members of loop
+ */
+function wp_find_hierarchy_loop( $callback, $start, $start_parent, $callback_args = array() ) {
+	$override = is_null( $start_parent ) ? array() : array( $start => $start_parent );
+
+	echo "wp_find_hierarchy_loop: $callback, $start, $callback_args\n";
+	if ( !$arbitrary_loop_member = wp_find_hierarchy_loop_tortoise_hare( $callback, $start, $override, $callback_args ) )
+		return array();
+
+	return wp_find_hierarchy_loop_tortoise_hare( $callback, $arbitrary_loop_member, $override, $callback_args, true );
+}
+
+/**
+ * Uses the "The Tortoise and the Hare" algorithm to detect loops.
+ *
+ * For every step of the algorithm, the hare takes two steps and the tortoise one.
+ * If the hare ever laps the tortoise, there must be a loop.
+ *
+ * @since 3.1
+ *
+ * @param callback $callback function that accupts ( ID, callback_arg, ... ) and outputs parent_ID
+ * @param $start The ID to start the loop check at
+ * @param array $override an array of ( ID => parent_ID, ... ) to use instead of $callback
+ * @param array $callback_args optional additional arguments to send to $callback
+ * @param bool $_return_loop Return loop members or just detect presence of loop?
+ *             Only set to true if you already know the given $start is part of a loop
+ *             (otherwise the returned array might include branches)
+ *
+ * @internal
+ *
+ * @return mixed scalar ID of some arbitrary member of the loop, or array of IDs of all members of loop if $_return_loop
+ */
+function wp_find_hierarchy_loop_tortoise_hare( $callback, $start, $override = array(), $callback_args = array(), $_return_loop = false ) {
+	$tortoise = $hare = $evanescent_hare = $start;
+	$return = array();
+
+	// Set evanescent_hare to one past hare
+	// Increment hare two steps
+	while (
+		$tortoise
+	&&
+		( $evanescent_hare = isset( $override[$hare] ) ? $override[$hare] : call_user_func_array( $callback, array_merge( array( $hare ), $callback_args ) ) )
+	&&
+		( $hare = isset( $override[$evanescent_hare] ) ? $override[$evanescent_hare] : call_user_func_array( $callback, array_merge( array( $evanescent_hare ), $callback_args ) ) )
+	) {
+		if ( $_return_loop )
+			$return[$tortoise] = $return[$evanescent_hare] = $return[$hare] = true;
+
+		// tortoise got lapped - must be a loop
+		if ( $tortoise == $evanescent_hare || $tortoise == $hare )
+			return $_return_loop ? $return : $tortoise;
+
+		// Increment tortoise by one step
+		$tortoise = isset( $override[$tortoise] ) ? $override[$tortoise] : call_user_func_array( $callback, array_merge( array( $tortoise ), $callback_args ) );
+	}
+
+	return false;
+}