If you refresh the page you have to start all over
If you are on a practice and you have to refresh the page it can be a nuisance because you have to start all over again. It would be a great feature if you did not have to start all over again if you refresh the page when you are on a practice. I got this idea from the site www.khanacademy.org. P.S. Keep up the amazing work duolingo team!
Not just refreshing, but if you hit the back button by accident you lose your progress as well. I've done this more than once by hitting the back button after viewing the sentence discussion page instead of the close button.