Index: wp-includes/functions-formatting.php
===================================================================
--- wp-includes/functions-formatting.php	(revision 2496)
+++ wp-includes/functions-formatting.php	(working copy)
@@ -446,7 +446,7 @@
 				$regex[2] .= '/';
 			} else {	// Push the tag onto the stack
 				// If the top of the stack is the same as the tag we want to push, close previous tag
-				if (($stacksize > 0) && ($tag != 'div') && ($tagstack[$stacksize - 1] == $tag)) {
+				if (($stacksize > 0) && ($tag != 'div') && ($tag != 'blockquote') && ($tagstack[$stacksize - 1] == $tag)) {
 					$tagqueue = '</' . array_pop ($tagstack) . '>';
 					$stacksize--;
 				}