Googlebot & Could not find valid value for id

Over the last few years every few days a fatal error is triggered by a Googlebot. Seems harmless and unnecessary but it is possibly affecting ranking.

The errorreport extension (ca.bidon.reporterror) reports:

HTTP_FROM: googlebot(at)googlebot.com
REDIRECT_QUERY_STRING:q=civicrm/event/register&snippet=4&type=
but also sometimes:
REDIRECT_QUERY_STRING:q=civicrm/event/register

The logfile in ConfigAndLog says:

May 17 19:16:31 [info] $backTrace = #0 /home/account/domains/ourdomain.nl/public_html/crm/sites/all/modules/civicrm/CRM/Core/Error.php(363): CRM_Core_Error::backtrace("backTrace", TRUE)
#1 /home/account/domains/ourdomain.nl/public_html/crm/sites/all/modules/civicrm/CRM/Utils/Request.php(95): CRM_Core_Error::fatal("Could not find valid value for id")
#2 /home/account/domains/ourdomain.nl/public_html/crm/sites/all/modules/civicrm/CRM/Event/StateMachine/Registration.php(52): CRM_Utils_Request::retrieve("id", "Positive", Object(CRM_Event_Controller_Registration), TRUE)
#3 /home/account/domains/ourdomain.nl/public_html/crm/sites/all/modules/civicrm/CRM/Event/Controller/Registration.php(47): CRM_Event_StateMachine_Registration->__construct(Object(CRM_Event_Controller_Registration), TRUE)
#4 /home/account/domains/ourdomain.nl/public_html/crm/sites/all/modules/civicrm/CRM/Core/Invoke.php(307): CRM_Event_Controller_Registration->__construct("Event Registration", TRUE, "null", NULL, "false")
#5 /home/account/domains/ourdomain.nl/public_html/crm/sites/all/modules/civicrm/CRM/Core/Invoke.php(86): CRM_Core_Invoke::runItem((Array:15))
#6 /home/account/domains/ourdomain.nl/public_html/crm/sites/all/modules/civicrm/CRM/Core/Invoke.php(54): CRM_Core_Invoke::_invoke((Array:3))
#7 /home/account/domains/ourdomain.nl/public_html/crm/sites/all/modules/civicrm/drupal/civicrm.module(489): CRM_Core_Invoke::invoke((Array:3))
#8 [internal function](): civicrm_invoke("event", "register")
#9 /home/account/domains/ourdomain.nl/public_html/crm/includes/menu.inc(519): call_user_func_array("civicrm_invoke", (Array:2))
#10 /home/account/domains/ourdomain.nl/public_html/crm/index.php(21): menu_execute_active_handler()
#11

I have 27 full examples of this now. Probably I'm not the only one experiencing this. What would be a cause of this? And what would be a start of a solution?

Also posted in http://civicrm.stackexchange.com/questions/2430/googlebot-could-not-find-valid-value-for-id