I just installed a fresh copy of wpmu 2.7.1 and got the following errors on various pages:
Warning: array_merge() [function.array-merge]: Argument #2 is not an array in /wp-includes/capabilities.php on line 537
Warning: array_merge() [function.array-merge]: Argument #1 is not an array in /wp-includes/capabilities.php on line 537
Warning: array_merge() [function.array-merge]: Argument #1 is not an array in /wp-includes/capabilities.php on line 539
Warning: Cannot modify header information - headers already sent by (output started at /wp-includes/capabilities.php:537) in /wp-includes/functions.php on line 689
Warning: Cannot modify header information - headers already sent by (output started at /wp-includes/capabilities.php:537) in /wp-includes/functions.php on line 690
First time I saw these errors was when trying to activate a user which I had created as "admin" in the backend. I think I added the new user to the list of SITE Admins which I guess might have caused confusions when activating...
Later I saw the same errors when adding the same user to a new blog using the "SiteAdmin > Blogs > Edit Blog" page... I was logged in as the new user at that time (does that matter?)
And another error which I don't remember which page it happened on:
Fatal error: Call to a member function has_cap() on a non-object in /wp-admin/users.php on line 59