What caused the 403 Forbidden Errors?

I upgraded a WP 2.9.2 site to WP 3.0.1
This is a barebones dev install with only a few posts/categories/tags.

The automatic upgrade seems to work ok.

But there was a problem.
At first I thought it was a Twenty10 problem, but it also happened with the old default theme, no plugins.
Took me quite a while to figure it out, I was getting 403 Forbidden errors.

I could run through the site and all pages/posts were loading, then one particular post would throw a 403 then all pages / posts that worked just fine would also throw a 403.

I deleted and renamed that post, the same scenario.

What worked was to delete that post and make a new one with a different month/date.
It seems the archive for Nov 2009 was what was causing the problem.
When I put a new post in Dec 2009 I no longer got the 403’s.

Long story short, I want to know why that happened?
As this is a test site, it wasn’t a big issue, but I do have other sites to upgrade and if this happens to them, well that wouldn’t be pretty. (they need to have their post dates correctly archived)

So any suggestions, things I should look for?
I did add a .htaccess file in the root as there wasn’t one, but that didn’t help either.

Thanks