Oops, here's the key information from the "desc" (description) 
attachment.

With cc-mode 1.55 I frequently got an error when refiling a message
using MH-E 8.0.3 after visiting a C source code file.  The Lisp stack
trace looked like this:

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  looking-at(nil)
  c-beginning-of-current-token()
  c-unfind-enclosing-token(38)
  c-before-change(1 38)
  erase-buffer()
  mail-extract-address-components("Janne Jokitalo 
<astraljava@kapsi.fi>")
  mh-folder-from-address()
  mh-prompt-for-refile-folder()
  (intern (mh-prompt-for-refile-folder))
  (list (mh-interactive-range "Refile") (intern (mh-prompt-for-refile-
folder)))
  call-interactively(mh-refile-msg)

This is with XEmacs 21.4.22.  I do not
see the problem with XEmacs 21.5.31+0df3cedee9ac, nor do I see it with
21.4.22 and the stable version of cc-mode (1.45 package).

The debugger tells me that the current buffer for the error is

  #<buffer " *extract address components*">

and its major-mode is fundamental-mode.

So it's odd that a C-mode function is getting invoked here.

If I do 

    M-: before-change-functions

in an MH-E folder buffer, I get different results for 21.4 and 21.5.

21.4 (cc-mode 1.55): (c-before-change)

21.4 (cc-mode 1.45): nil

21.5 (cc-mode 1.55): nil

I don't know where the bug is here, so I'm filing against cc-mode to
start.

With cc-mode 1.57 I got a similar error when trying to insert an <h4>
line in an HTML file using psgml (C-c C-e h4 RET).