I've just upgraded to 2.6, and started getting lots of 404 errors in my logs since. Lots of posts on here about people having the same problem when they are using index.php? style URLs, but I'm not.
For example - the post with "Goldfish" in the title on this page is a valid post, but clicking on it's title 404's (effectively):
Clicking on the posts on either side of it work however. Very annoying!
The posts formats look a bit odd as they were imported from a previous piece of blogging software, but I don't think thats the problem, as many of the posts that I imported work fine.
Anybody got any ideas?