ACP/Manage users/Find a member
There're "Select" links under usernames. But if you go next pages, you'll get checkboxes instead of "Select" links. When you return to the first page then, you'll get checkboxes on it as well.
Looks like a bug because you can select only one user to manage.

Possible solution: http://phpfi.com/302573 (Thanks for Schumi)