Please prompt us before letting us leave the lesson
All too many times I have hit the backspace key when I'm not in a text box, and my browser goes back to the skill screen. This gets very irritating, please fix it.
Similarly, on numerous occasions I have accidentally clicked on a username in a sentence discussion. This takes me to a user page and appears to delete all my progress in the lesson. Not very nice. Well, I just take it as additional exercise...
EDIT: Looking at other discussions such as http://www.duolingo.com/comment/673737?, there seems to be a prompt for this situation, I'm just not getting it for some reason.
This is an old convention that really cannot be blamed on Duolingo. Depending on your browser, different solutions are available.
For Chrome, the easiest would be to download this extension http://www.chromeextensions.org/appearance-functioning/backspacemeansbackspace/
That bassicly removes the page 'back' functionality. If you are using a different browser, do notify.