When using the username search (in ACP, memberlist, PM recipients, add friends/foes...) the results are always filtered to only bring those of user_type USER_NORMAL and USER_FOUNDER. While this is good for normal users, administrators should be able to find all users. I've encountered at least two topics in the support forums where users were unable to find usernames, just because those were USER_INACTIVE.
To overcome this, the search should omit the filter on user_type if the current user has administrator permissions. The changes would affect /memberlist.php only: for both occurances change