Reports of corrupt mail on mail fetch

[Harry Putnam <reader@newsguy.com>]

Running a fairly recent v5.10.3.

Lately I've been getting a message during mail/news fetch of corrupt
unrecognized mail format:

nnml: Reading incoming mail from directory...
Mail source (directory :path /home/reader/spool/ :suffix .in) error (Error, unknown mail format! (Possibly corrupted buffer ` *nnmail incoming*'.)).  Continue? (y or n) 

Doesn't occur on every fetch, or even most.  But still fairly often. 

Checking ~/.emacs-mail-crashbox*, there will be a file with 1-5 bytes
reported as size.  1-5 newlines inside.  Deleting that file cures the
problem for a while.

The report doesn't seem to indicate what actual spool is involved.
Unless it means the Inc* file which I have turned off, and have had
it off for a very long time.  Maybe I should turn it back on and see
if something unusual is coming down the pipe.

I thought maybe a renegade procmail recipe might do this.  I change
procmailrc very often.  But reviewing verbose procmail log output
I noticed nothing to catch my attention.

Is there some way I can force gnus to give more complete info? Or in
some way make a connection between actual incoming mail and those
newlines? 

I guess setting debug on error until it happens again might get a
backtrace, so trying that for now.