On line 1656 of classes/class-s2-core.php in the subscribe2 plugin, there's a reference to the s2_admin class (or, presumably, the s2_frontend class, depending on the user level):
add_action('init', array(&$this, 'wpmu_subscribe'));
yielding (with—in our case—the s2_admin class) this error:
Warning: call_user_func_array() expects parameter 1 to be a valid callback, class 's2_admin' does not have a method 'wpmu_subscribe' in /mnt/base/www/wp-includes/plugin.php on line 403
when I believe it should be calling the function on the s2_multisite global:
add_action('init', array(&$s2class_multisite, 'wpmu_subscribe'));
