As a result of MIT's class naming scheme, I have several folders of
the form nnml:mail.classes.6.001, ...6.170, and so on.  I also have a
general folder nnml:mail.classes.6 which is used for an announcement
list for all things pertaining to the department of Electrical
Engineering and Computer Science (Course 6), which is total-expirable.

I noticed today (using pgnus 0.95 under XEmacs 20.4) that Gnus was
trying to expire ~/Mail/mail/classes/6/001, even though that's a group 
directory rather than an nnml message file.  This is because
nnheader-directory-articles doesn't look to see whether the files
nnheader-directory-files-safe returns are directories or not.  This
can be fixed by tweaking the arguments passed to directory-files,
which the attached patch to nnheader.el does.