Mail source problem (?)

[Jeremy Hankins]

Recently (after an update to emacs about a month ago, debian unstable) I
started getting a message when checking mail about a mail source error:

Mail source (file :path /var/mail/nowan) error (Searching for program). 
Continue? (yes or no)

This is apparently from my secondary select method meant to pull in
localhost mail, and it doesn't seem to be anything obvious like a
permissions problem.  It doesn't make any difference whether I respond
yes or no, either way mail out of /var/mail is ignored and the rest of
my mail is fetched successfully.

It's clearly some sort of compatibility issue and I figured I needed to
sort through my gnus file and figure out where it was coming from --
annoying but no big deal.  And since it's only a small minority of my
mail affected it hasn't bugged me enough to pursue it till now.

But the annoying thing, apart from the fact that the error is a bit
obscure (what program?), is that if I turn debug-on-quit on so as to
figure out where the bug is being triggered and why, the message goes
away removing my chance to trigger the debugger.

Any ideas either a) what the problem fetching local mail is, or b) why
debug-on-quit makes the message go away?

-- 
Jeremy Hankins <nowan@nowan.org>