The only time this happens is when the theme is changed, and it's the first time back to the site, least as far as I can tell.

If you click on one of the links in a reply posted email, sometimes it says that the topic doesn't exist.

If you look through the forums you can find the topic.

I've seen that happen too. I got one email last week like that. The last number in the link was wrong. :?

mikem