Bug in magento 1.6.x

There is an issue in Magento 1.6.x which leads to errors when reindexing.

To fix this you have to execute after every import following SQL:

 UPDATE catalog_product_entity_varchar u
 SET value = (
   SELECT GROUP_CONCAT(o.option_id)
   FROM eav_attribute_option o
   WHERE 
     u.attribute_id = o.attribute_id AND
     FIND_IN_SET(option_id, u.value) > 0
   ORDER BY o.sort_order
 )
 WHERE
   u.attribute_id IN (
     SELECT attribute_id FROM eav_attribute 
     WHERE frontend_input ='multiselect' AND backend_model ='eav/entity_attribute_backend_array'
   )

Credits go to http://www.mash2.com/ for initially creating this query and sharing it