Mouseover moves the underlying word.
I'm using Firefox. In this translation : http://duolingo.com/#/translation/eb0fb3b45beea620863559f57aada84a?index=10=cda61ef8df36064689367a35811dfc40
If you mouse over the word "polvo" in the second last sentence, it moves back to the previous line, and then the mouse over disappears, and the word moves back, and again and again.
Maybe you can look into why it's rendering this way.