(flyspell-external-point-words):

Detect when WORD can't be checked properly because
flyspell-get-word finds just part of it, and move on.

lisp/textmodes/flyspell.el

@@ -1356,7 +1356,14 @@ The buffer to mark them in is `flyspell-large-region-buffer'."
 		  (progn
 		    (setq flyspell-large-region-beg (point))
 		    (goto-char (- (point) 1))
-		    (setq keep (flyspell-word)))
+		    (setq keep
+			  ;; Detect when WORD can't be checked properly
+			  ;; because flyspell-get-word finds
+			  ;; just part of it, and treat that as ok.
+			  (if (< (length (flyspell-get-word following))
+				 (length word))
+			      nil
+			    (flyspell-word))))
 		(error "Bug: misspelled word `%s' (output pos %d) not found in buffer"
 		       word wordpos)))))))
     ;; we are done