Index: update-core.php
===================================================================
--- update-core.php	(revision 12549)
+++ update-core.php	(working copy)
@@ -227,6 +227,9 @@
 	$php_compat     = version_compare( $php_version, $required_php_version, '>=' );
 	$mysql_compat   = version_compare( $mysql_version, $required_mysql_version, '>=' ) || file_exists( WP_CONTENT_DIR . '/db.php' );
 
+	if ( !$mysql_compat || !$php_compat )
+		$wp_filesystem->delete($from, true);
+
 	if ( !$mysql_compat && !$php_compat )
 		return new WP_Error( 'php_mysql_not_compatible', sprintf( __('The update cannot be installed because WordPress %1$s requires PHP version %2$s or higher and MySQL version %3$s or higher. You are running PHP version %4$s and MySQL version %5$s.'), $wp_version, $required_php_version, $required_mysql_version, $php_version, $mysql_version ) );
 	elseif ( !$php_compat )