bug-21824.el

;;; emacs -Q -l bug-21824.el -f 21824-trigger

(defun 21824-mod-overlay (overlay afterp beg end &optional len)
  (if (eq (current-buffer) (overlay-buffer overlay))
      (message "21842-mod-overlay(%s %s %s %s %s), buf = %s"
               overlay afterp beg end len (current-buffer))
    (setq 21842-repeat-message)
    (debug nil overlay afterp beg end len (current-buffer))))

(defun 21824-trigger ()
  (switch-to-buffer "foo.cpp")
  (view-echo-area-messages)
  (erase-buffer)
  (insert "xyy")
  (search-backward "yy")
  (let* ((beg (match-beginning 0))
         (end (match-end 0))
         (mod-hook-ov (make-overlay beg end nil nil t)))
    (overlay-put mod-hook-ov 'face 'highlight)
    (overlay-put mod-hook-ov 'modification-hooks '(21824-mod-overlay))
    (goto-char (1- end))
    (insert "y")) ; trigger modification-hooks

  (with-current-buffer "*Messages*"
    (make-overlay 1 2))

  (message "a message")
  (message "a message"))