Arisia has, as some of you know, a database. This particular database gets updated when people buy memberships, and also when we get changes of address from the post office. That's the input side. Then the database is used a few months before the con to send 4000 or so people a mailing saying in broad terms what will be going on and why they should buy a membership, and again a few weeks before the con to send 1000 or so people a mailing saying in more specific terms a few of the goings-on that they might need to know about to plan their trip and confirming that they have in fact got a membership. That's the output side.
A side-effect of sending out the big mailing is that we get a lot of address updates. If you look at a small sample of address updates, you might find that they are for people who haven't been to the con for a while and haven't had a chance to update their addresses themselves. So it's easy to imagine that it doesn't matter when you get the address updates back into the system, so long as it happens before the next time you send out a big mailing. In fact, you might imagine that it's not necessary to have all the bits in order at the post office to even get the address updates until the next big mailing. But in fact a fair number of updates are from people who do have memberships, but have moved since the previous con. And some of these are reflected in bounces have only recently come unjammed from the post office pipeline.
Lessons for next time: in addition to automated NCOA feedback (which we knew was a problem), make sure that the return mail permit has $350 or so in it before sending out the mass mailer. It matters, even if you think it doesn't.