um-mode must inhibit ro because special-mode already is, i think

um.el

@@ -456,13 +456,14 @@ Optional TAGS string may contain more than one tag separated by a comma.
 \\{um-mode-map}
 "
   ;; restrict movement to (bolp) with cursor-intangible-mode
+  (let ((inhibit-read-only t))
     (save-excursion
       (goto-char (point-min))
       (while (not (eobp))
         ;; NOTE: actually the newline is the only position not included, but this
         ;; positions the cursor visually at the bol:
         (put-text-property (pos-bol) (pos-eol) 'cursor-intangible t)
-      (forward-line 1)))
+        (forward-line 1))))
   (cursor-intangible-mode)
   (read-only-mode))