After updating to the latest BPS, on a multisite WP installation v. 3.5.1, admin pages immediately became unavailable, due to the following error:
PHP Fatal error: Call to undefined function wp_get_current_user() in /home/apache/wordpress/wp-includes/capabilities.php on line 1451
when this happens it is because wp-includes/pluggable.php contains this function, but this file is not getting loaded until after the plugins are loaded.
I was able to fix the problem by adding:
require_once(ABSPATH . 'wp-includes/pluggable.php');
as soon as I added that line admin pages became accessible again and the error is no longer triggered.