My home-grown process for building mail spool files for gnus produces
the attached spool. When gnus slurps this up

(setq mail-sources
      '(
	(file
	 :path "~/.incoming/mail.spool"
	 :plugged t)))

it attaches the putatively second message in the spool onto the first
(and because of other settings I've got, it's completely invisible to me.)

Bug in my spool construction process?
Bug in gnus reading the spool?

What can I do to make sure this never, ever happens again. I came
"this close" to missing a really important message, due to this bug.