After selecting an existing event, editing it, and saving it, the following notice appears:
Notice: Undefined property: wpdb::$update_id in [...]/wp-includes/wp-db.php on line 566
This is lines 199-201 of simple-events-calendar.php:
$wpdb->update( $table_name, $editEvent, $where ); $result = $wpdb->update_id; echo '<div id="message" class="updated fade"><p>'.__('The event was succesfully updated!',SE_TEXTDOMAIN).'</p></div>';
$wpdb does not set an update_id in the update() method, and anyhow it appears that $result is not being used, so this line should probably just be removed.
In other, related news: the word "successfully" is misspelled on line 201.