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.