Incorrect validation test in og_is_group_admin().

I believe a $node object needs to be passed to og_is_group_admin and not $nid. og_is_group_admin is expecting to test $node->nid and $node->type.

function og_announce_validate(&$form, &$form_state) {
  // Allow only group admins to post to announcement only groups.
  foreach ($form_state['values']['og_initial_groups'] as $key => $nid) {
    if (og_announce_is_announce($nid) && !og_is_group_admin($nid)) {
      form_set_error('og_groups', t('Only group administrators may post to this group.'));
    }
  }
}

The og_is_group_admin function:

function og_is_group_admin($node, $account = NULL) {
  if (empty($account)) {
    global $user;
    $account = drupal_clone($user);
  }

  // Modules can cause us to arrive here before og_init() has fired.
  // See http://drupal.org/node/285696
  if ($account->uid && !isset($account->og_groups)) {
    // Reload the user object.
    $account = user_load($account->uid);
  }

  return og_is_group_type($node->type) && (user_access('administer nodes', $account) || !empty($account->og_groups[$node->nid]['is_admin']));
}