If you are running a shared-login setup, then users who sign up on one site can
not be made sysops on a wiki other than the one they signed up for. The reason
is that the Makesysop.php script uses a left join statement to get the user's id
from the shared user table and their rights from the local user_rights table.
However, the user_rights table entry is made when the user signs up. So, only
one site will have a user_rights entry in it's table for the user. So, the
query turns up null, and any attempt to make the user a sysop results in a query
trying to add sysop rights to the user with id 0.
I've fixed my setup by splitting the join into two queries, so that I always
have the correct user id, regardless of what's in user_rights.