7 calls to drupal_is_denied()

user_admin_access_check_submit in drupal-6.x/modules/user/user.admin.inc
user_authenticate in drupal-6.x/modules/user/user.module
Try to log in the user locally.
user_login_name_validate in drupal-6.x/modules/user/user.module
A FAPI validate handler. Sets an error if supplied username has been blocked or denied access.
user_pass_reset in drupal-6.x/modules/user/user.pages.inc
Menu callback; process one time login link and redirects to the user page on success.
user_pass_validate in drupal-6.x/modules/user/user.pages.inc
_drupal_bootstrap in drupal-6.x/includes/bootstrap.inc
_user_edit_validate in drupal-6.x/modules/user/user.module