Suggestion: "Restart" button in a lesson.
I was thinking, after I quit a lesson, I thought 'Why don't they make a restart button?'. It could look like this:
Most people just use the "Reload Current Page" icon in their browser to restart a lesson.
Yeah, it does seem better that quit, like when I have a typo in my second answer or forgot to turn off the microphone option in class. (Not that I use Duolingo to kill time in boring lectures or anything! o.o)
A restart button would be nice for the mobile versions particularly. I hate when I make some stupid mistake right off the bat, "Well, I guess I'll just go ahead and do this again now."