I just upgraded my blog to WordPress 2.5 RC2 and everything stopped working. Instead of getting my blog or the admin screen, I got the following error message:

Fatal error: Call to a member function add_query_var() in taxonomy.php

I discovered the root-cause of the problem and it is the wonderful Simple Tags plugin. Disabling the plugin restores WordPress back to it’s own awesome self. Simple Tags is an awesome plugin that allows you to manage tags within WordPress.

I’ve opened an issue with the Simple Tags issue tracker on Google Code.