spamassassin very frequently has problems with some messages I receive:

Apr 25 00:55:44 lazarus spamd[1803]: connection from lazarus [127.0.0.1] at 
port 34133
Apr 25 00:55:44 lazarus spamd[5290]: info: setuid to clambin succeeded
Apr 25 00:55:44 lazarus spamd[5290]: processing message 
<200304242255.h3OMtfc9005285> for clambin:500, expecting 
26044 bytes.
Apr 25 00:55:44 lazarus spamd[5290]: bad protocol: header error: (Content-
length mismatch: 26044 vs. 25749)
Apr 25 00:55:44 lazarus sendmail[5286]: h3OMtfc9005285: 
to=<clambin@localhost>, delay=00:00:00, xdelay=00:00:00, mailer=local, 
pri=55974, dsn=2.0.0, stat=Sent

Such a message seems to be simple dropped: it never appears in my mbox file.  
If my mbox is empty, I've also noticed that a blank line will be in added to 
it. Such a blank line makes mutt fail to open it.

Versions of related components:
spamassassin-2.44-11.8.x
procmail-3.22-9
sendmail-8.12.8-5.90

My /etc/procmailrc:
DROPPRIVS=yes

:0fw
| spamc

:0e
EXITCODE==$?

:0:
* ^X-Spam-Flag: YES
$HOME/Mail/junk