This bug originally arose using message-mode to forward a message with
mml markup "type=message/rfc822 disposition=inline"

Unfortunately I don't have permission  to share the test message, but
the failure of the corresponding line in message-goto-body can be
duplicated as follows.

1) start emacs -Q
2) visit the attached file
3) run M-: (search-forward-regexp "[^:]+:\\([^\n]\\|\n[ \t]\\)+\n\n" nil t)

This is an anonymized list of a couple hundred recipients. I agree
that's not typical, but it's not that large either.