Index: wp-includes/shortcodes.php
===================================================================
--- wp-includes/shortcodes.php	(revision 26748)
+++ wp-includes/shortcodes.php	(working copy)
@@ -262,7 +262,7 @@
 
 	// allow [[foo]] syntax for escaping a tag
 	if ( $m[1] == '[' && $m[6] == ']' ) {
-		return substr($m[0], 1, -1);
+		return '['.substr($m[0], 2, -1);
 	}
 
 	$tag = $m[2];
@@ -379,7 +379,7 @@
 function strip_shortcode_tag( $m ) {
 	// allow [[foo]] syntax for escaping a tag
 	if ( $m[1] == '[' && $m[6] == ']' ) {
-		return substr($m[0], 1, -1);
+		return '['.substr($m[0], 2, -1);
 	}
 
 	return $m[1] . $m[6];