Some users (people with doanything capability is most common case, but not only them) may have both mod/quiz:preview and mod/quiz:attempt allowed.
For them, quiz works extremly strange. view.php show a button with title "attempt quiz now", but when you press it you actually start a preview... This is obvious inconsistency and should be solved somehow (to avoid printing two buttons you could use button to start attempt and link in a quiz administration block to start preview for example) - at the very least logic managing button title should be corrected.
To see it login as admin to you testing site and try to attempt a quiz.