My apache log keep throwing an error for this module and after looking at the code I suspect that when user is anonymous (uid = 0) the module didn't check it.

After modifying :

// line 289
  if ($account->uid != 0) {
      $signature = db_result(db_query("SELECT signature FROM {users_signature} WHERE uid = %d", $account->uid));
      // Bug #190446 OG puts $account->signature into comments
      $account->signature_forum = $signature;
  }

// line 559
if ($load_signature == TRUE && $al->uid != 0) {

the error went away.

Hopefully someone can create & test proper patch for this.