I have two accounts and I've set them up to forward to one another just
as in the procmailex man page.
A while ago one of the accounts failed (I'm still not clear on exactly
what failed; I think the NFS server with the mail spool files died).
Anyway, a single email to the other account was forwarded; the
mailer-daemon on the broken account sent back a response which included
the entirety of the original message; this was forwarded back, etc,
etc, etc. This N^2 process continued until, several gigabytes later,
I'd severely impaired the main mailserver for my entire university and
the sysadmin forcibly removed my procmail filter.
Although the problems with the broken account have supposedly been
fixed, I wonder if there is a way to prevent such a fiasco in the
procmail mailing list