bug#728: 23.0.60; flyspell checking is sometimes silently disabled

[Markus Triska <markus.triska@gmx.at>]

Sometimes in flyspell-mode, the mode line still says "Fly", but no
more flychecking is actually performed. I cannot yet reproduce it
reliably; however, if you add the following to your .emacs:

   (defun my-flyspell-check ()
     (when flyspell-mode
       (unless (memq 'flyspell-post-command-hook post-command-hook)
         (with-current-buffer (get-buffer-create "flywarn")
           (insert "flychecking inactive despite flyspell-mode!\n"))
         (display-buffer  "flywarn"))))

   (setq flycheck-timer (run-with-timer 0 0.5 'my-flyspell-check))

you will be informed when flyspell mode is superficially active, but
no checking is actually performed. I only encountered the problem once
since I have the above code in my .emacs, and then I also saw:
"Invalid search bound (wrong side of point)" in *Messages*, which I
previously probably missed. Thus I think this could for example happen
when switching buffers or using the minibuffer at the wrong time.

It is clear that flyspell-post-command-hook must be removed from
post-command-hook in case of errors; if the error is hard to catch in
the hook, I would already find it an improvement if flyspell mode were
cleanly disabled (existing underlines, mode line, ...) in such cases.

In GNU Emacs 23.0.60.1 (i386-apple-darwin8.11.1, GTK+ Version 2.12.9)
 of 2008-08-15 on mt-computer.local
Windowing system distributor `The XFree86 Project, Inc', version 11.0.40400000
Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: nil
  default-enable-multibyte-characters: t