I've just upgraded to WordPress 2.6. As I have claims to be a technologist I install and upgrade my blog software using Subversion. An upgrade simply requires issuing an 'svn switch' command and then running the WordPress database upgrade via the page that presents. Usually this works like a charm. Today I had issues.
I've logged a support ticket but thought I should mention it here in case anyone else sees the same issue.
When I tried to log in after the database upgrade the login page just kept re-directing back to itself. The main page of this blog was showing, but with an error message stating (in part) - "error in akismet.php on line 487".
After a little checking around I discovered that Akismet (the WordPress spam fighting plugin) needed to be upgraded at the same time as the core WordPress code and hadn't. The old version (2.1.4) is not compatible with the latest WordPress release and needed to be upgraded (to 2.1.6). A simple workaround was to download and install the latest version of Akismet by hand so it wasn't a huge problem, but it would have been nice if the WordPress Subversion repository had been updated to reflect the change.
Update: OK, it's probably me. A fresh checkout of WordPress (release 2.6 or 2.5) comes complete with the correct version of the Akismet plug in. It must have a problem with my 'svn switch' that I didn't catch.