FS#4286 brought to light an interesting failure mode for mail delivery. If clamscan times out (because clamscan is incredibly, astonishingly, and embarrassingly slow on Debian 4.0), mail is delivered to /dev/null, as though it returned a "virus" response. Thus, on a system with an incredibly slow clamscan, mail will always be delivered to /dev/null...no bounce, no warning except what's in the procmail.log. On systems that have almost too slow clamscan, this could presumably happen intermittently.
So, failure mode should be delivery or bounce...never /dev/null.