Very strange failmail just came through, looks like something in the session was stale, which is consistent with the fact that it came in hours after the campaign went down. No other log entries for this ID, though.
This one came in through /job/Donations queue consume/375435/consoleFull and by that point, payment_method was empty and payment_submethod=cb.
In the payments logs, at Oct 19 21:55:26.