I trace down to the while loop in nxml-forward-single-balanced-item:

(defun nxml-forward-single-balanced-item ()
  (condition-case err
      (goto-char (let ((end (nxml-token-after)))
   (save-excursion
     (while (eq xmltok-type 'space)
       (goto-char end)
       (setq end (nxml-token-after)))
     (cond ((/= (point) xmltok-start)

In my test xml file, when the last comment char is deleted, `end' will be
65, then (nxml-token-after) also returns 65  , so this is a endless loop.