1. Go to item list and create a log item.
2. In item list view open trigger wizard form and enter data, so that everything is valid.
3. Press "Add".
4. After trigger has been created and popup window closed, the previous item edit form is trying to submitted again. You can see the message on FF, for example: "To display this page, Firefox must send information that will repeat any action (such as a search or order confirmation) that was performed earlier."

Additionally Zabbix 3.0 doesn't close the popup window.