Because this will result in UI problems - this will be visible to the user. You probably should handle StaleObjectException in the DAO, rethrow it as some checked exception and somewhere on top near UI layer we need to show warning and reload the page.
There should be some investigation carried out. For instance we need to figure out how to handle such errors globally if it's possible. Or how to show a dialog and after pressing Okay reload the page.

Maybe let's think about different way.. We can implement that if we get an error, we show in on the screen and after pressing Ok, the page is refreshed and we're showing the last visited page.
This would depend on other stories, like remembering ans opening last visited page and also showing an error dialog that can refresh the whole page.

Actually I've expected something like Stanistav described in comment above: error message appears and page is refreshed after clickin OK. Is it possible?

I am trying to implement this. But on step "refresh page" there are many ways. I see only two: open forum structure window(but I think there may be old structure), and refresh whole page(but in this case user will must to click on components, on components, than on forum, and only than he will see updated structure)

Let's do it gradually. First of all we need a Dialog window that should refresh the page if unexpected errors appear. Nickolay Polyarniy, could you raise a separate JIRA for that? You might want to look at how to configure that window globally for errors in ZK, I bet there should be something for that in zk.xml.