I have written some code that makes the first line read-only:

(save-excursion
  (goto-char (point-min))
  (set-text-properties (point-at-bol) (point-at-eol) '(read-only t))
  )

When I try to remove all text-properties:

(condition-case err
    (save-excursion
      (set-text-properties (point-min) (point-max) nil))
  (error
   (message "dmp-error:err=%s" err)))

It barfs with the following error message:

dmp-error:err=(text-read-only)