I very recently switched to using getmail rather than fetchmail to get email from a POP3 account. I use procmail as the local MDA so I can call SpamAssassin.
It kept thinking there was an error in the mail delivery and exiting without deleting the mail, even though it got delivered.
Turns out one *really* ought to set LOGFILE at the top of one's .procmailrc, since getmail sees some stderr output from procmail and assumes an error, even if the exit code is 0.
I got the hint from this thread. Seems to me this perhaps ought to be in the FAQ for getmail, so I'll see about submitting it. Granted, it's a user error in configuring procmail, but still.