It's been brought to my attention that OpenBSD 3.2 can assign the same pid to different processes in the same chronological second (that is, one process terminates, and its pid is immediately assigned to a new process that's created soon thereafter).

Since filenames for messages in maildirs are generated based on the combination of the pid, and the current time, in seconds, there is now a race condition that will result in loss or corruption of mail. This breaks Courier and Qmail. Example: process A delivers a message to a maildir just before process B reads Maildir/new, and A's message is moved to Maildir/cur a short time after it's been delivered, and A exits; meanwhile process C starts, and delivers mail to the same maildir; its generated filename will now be the same; so now you will have two messages with the same base filename in the maildir).

This is not theoretical. This been brought to my attention after someone managed to succesfully deliver two messages with the same filename, on a piddly Athlon (as part of a Courier "stress test"). Let's have a round of applause for such a noteworthy accomplishment!