"Are you sure you want to leave this lesson?"
Here's my biggest pet peeve about Duolingo: you're halfway complete through a lesson, you accidentally click a link, and the browser just goes to that link. You lose all progress.
Far too many times have this happened to me. It's extremely annoying.
Duolingo should have some kind of dialog confirming if you really want to leave a lesson, instead of going to another page with no warning.
Surely that will cut down on computer abuse by Duolingo users...