Your cron will run at specified intervals, such as every 5 minutes.
Magento, will throw that message if any block has been modified. This can be from products, to CMS, or anything that is in a cached block. This is expected behavior you are experiencing.
You can look here for some reasons:
Mage::app()->getCacheInstance()->getInvalidatedTypes();
That is found in this class
Mage_Core_Model_Cache
/**
* Get array of all invalidated cache types
*
* @return array
*/
public function getInvalidatedTypes()
{
$invalidatedTypes = array();
$types = $this->_getInvalidatedTypes();
if ($types) {
$allTypes = $this->getTypes();
foreach ($types as $type => $flag) {
if (isset($allTypes[$type]) && $this->canUse($type)) {
$invalidatedTypes[$type] = $allTypes[$type];
}
}
}
return $invalidatedTypes;
}
There are many reason why a block may be invalidated, I guess I would not worry too much about it. However, if your customer will eventually not just take our word that things are working fine, you can create an observer that fixes the invalid blocks when an admin user logs in the site. That seems a bit much but I have had non technical customers before and sometimes its easier just to find a fix rather than try to explain the depths of Magento to a store owner.