Pressing delete in the middle of a lesson may lose your progress b/c browser triggers back button
Depending on the situation, pressing delete/backspace may trigger the browser to go back, which means you'll lose your progress. Since we're typing, there's no reason why we wouldn't be using the delete key. Seems like this could easily be solved by causing an alert to popup saying "You're in the middle of a lesson, are you sure you want to go back?" similar to how Facebook alerts you if you're in the middle of a post. I've had this problem since the beginning of duolingo, and I'm surprised they still haven't done anything about it.
I can not tell you how many times I have done that. The most aggravating is when you are on the last question and suddenly you are back to the duolingo tree. I guess that is when you scream nooooooo!