Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 18515)
+++ wp-includes/functions.php	(working copy)
@@ -1037,6 +1037,8 @@
 	if ( is_array( $data ) || is_object( $data ) )
 		return serialize( $data );
 
+	// Double serialization is required for backward compatibility.
+	// See http://core.trac.wordpress.org/ticket/12930
 	if ( is_serialized( $data ) )
 		return serialize( $data );