Running M-q in a draft message with orgstruct++-mode enabled gives me the appended backtrace.
The problem seems to have been introduced by commit 84a358e3. Git blame fingers the following two as the most recent commits that affected org-fill-paragraph: 187f77a2 2012-04-04 84a358e3 2012-04-26 With the first one in force, I don't get the problem; with the second, I do. Nick Debugger entered--Lisp error: (wrong-number-of-arguments #[(arg) ("/usr/local/share/emacs/24.0.95/lisp/mail/sendmail.elc" . 26697) nil 4] 0) mail-mode-fill-paragraph() funcall(mail-mode-fill-paragraph) (cond ((and (equal (char-after (point-at-bol)) 42) (save-excursion (goto-char (point-at-bol)) (looking-at org-outline-regexp))) t) (table\.el-p t) (table-p (org-table-align) t) (itemp (let* ((struct (save-excursion (goto-char itemp) (org-list-struct))) (parents (org-list-parents-alist struct)) (children (org-list-get-children itemp struct parents)) beg end prev next prefix) (cond ((not children) (setq prefix (make-string (org-list-item-body-column itemp) 32) beg itemp end (org-list-get-item-end itemp struct))) ((< (point) (setq next (car children))) (setq prefix (make-string (org-list-item-body-column itemp) 32) beg itemp end next)) ((> (point) (setq prev (car ...))) (setq beg (org-list-get-item-end prev struct) end (org-list-get-item-end itemp struct) prefix (save-excursion (goto-char beg) (skip-chars-forward " ") (make-string ... 32)))) (t (catch (quote exit) (while (setq next ...) (if ... ... ... ...))))) (flet ((fill-context-prefix (from to &optional flr) prefix)) (sav e-restriction (narrow-to-region beg end) (save-excursion (fill-paragraph justify)))) t)) ((and (derived-mode-p (quote org-mode)) (save-excursion (forward-paragraph -1) (setq itemp (org-in-item-p)))) (let ((struct (save-excursion (goto-char itemp) (org-list-struct)))) (save-restriction (narrow-to-region (org-list-get-bottom-point struct) (save-excursion (forward-paragraph 1) (point))) (fill-paragraph justify) t))) ((and (eq major-mode (quote org-mode)) (save-excursion (forward-paragraph -1) (or (looking-at (concat "^[^\n]*" org-scheduled-regexp ".*$")) (looking-at (concat "^[^\n]*" org-deadline-regexp ".*$"))))) (save-restriction (narrow-to-region (1+ (match-end 0)) (save-excursion (forward-paragraph 1) (point))) (fill-paragraph justify) t)) (org-fill-paragraph-fallback-function (funcall org-fill-paragraph-fallback-function)) (t nil)) (let ((table-p (org-at-table-p)) (table\.el-p (org-at-table\.el-p)) (itemp (org-in-item-p))) (cond ((and (equal (char-after (point-at-bol)) 42) (save-excursion (goto-char (point-at-bol)) (looking-at org-outline-regexp))) t) (table\.el-p t) (table-p (org-table-align) t) (itemp (let* ((struct (save-excursion (goto-char itemp) (org-list-struct))) (parents (org-list-parents-alist struct)) (children (org-list-get-children itemp struct parents)) beg end prev next prefix) (cond ((not children) (setq prefix (make-string ... 32) beg itemp end (org-list-get-item-end itemp struct))) ((< (point) (setq next ...)) (setq prefix (make-string ... 32) beg itemp end next)) ((> (point) (setq prev ...)) (setq beg (org-list-get-item-end prev struct) end (org-list-get-item-end itemp struct) prefix (save-excursion ... ... ...))) (t (catch (quote exit) (while ... ...)))) (flet ((fill-context-prefix (from to &optional flr) prefix)) (save-restriction (narrow-to-region beg end) (save-excursion (fill-p aragraph justify)))) t)) ((and (derived-mode-p (quote org-mode)) (save-excursion (forward-paragraph -1) (setq itemp (org-in-item-p)))) (let ((struct (save-excursion (goto-char itemp) (org-list-struct)))) (save-restriction (narrow-to-region (org-list-get-bottom-point struct) (save-excursion (forward-paragraph 1) (point))) (fill-paragraph justify) t))) ((and (eq major-mode (quote org-mode)) (save-excursion (forward-paragraph -1) (or (looking-at (concat "^[^\n]*" org-scheduled-regexp ".*$")) (looking-at (concat "^[^\n]*" org-deadline-regexp ".*$"))))) (save-restriction (narrow-to-region (1+ (match-end 0)) (save-excursion (forward-paragraph 1) (point))) (fill-paragraph justify) t)) (org-fill-paragraph-fallback-function (funcall org-fill-paragraph-fallback-function)) (t nil))) org-fill-paragraph(nil) fill-paragraph(nil t) call-interactively(fill-paragraph) (let (orgstruct-mode) (call-interactively (or (key-binding "\361") (quote orgstruct-error)))) (if (org-context-p (quote headline) (quote item) (and orgstruct-is-++ nil (quote item-body))) (org-run-like-in-org-mode (quote fill-paragraph)) (let (orgstruct-mode) (call-interactively (or (key-binding "\361") (quote orgstruct-error))))) orgstruct-hijacker-command-22(1) call-interactively(orgstruct-hijacker-command-22 nil nil)