diff --git a/booking.variety_admin.inc b/booking.variety_admin.inc
index b953d80..e126587 100644
--- a/booking.variety_admin.inc
+++ b/booking.variety_admin.inc
@@ -763,6 +763,11 @@ function booking_variety_regn_edit_form_submit($form, &$form_state) {
     $previous_variety_ids = drupal_json_decode($person->booking_variety_ids);
 
     foreach ($previous_variety_ids as $previous_tid => $previous_sid) {
+      if (! isset($booking_variety_ids[$previous_tid])) {
+        watchdog('booking_debug', 'Person @nid in timeslot @tid is no longer registered for a variety session', array(
+          '@nid' => $values['nid'], '@tid' => $previous_tid
+        ));
+      }
       if ($booking_variety_ids[$previous_tid] != $previous_sid) {
         // reduce regn count for old session
         db_update('booking_variety_sessions')