Steps to reproduce:

1. Go to Administration => Script
2. Unselect all items
3. Go to Administration => Media Types
4. Select several items
5. Go to Administration => Script again and look to selected check-box count in left down corner (selected-items-indicator.png)

Implementation notes:
Each list page which have zabbix.php?action={action-name} link should have separate cb_zabbix_{suffix} cookie name to store selected items.
Page itself should delete cookie only related to that list page.