Make it harder to leave a practice session by accident
Also, during timed practice, the timer should pause when you go to view that question's discussion. Sometimes I'm just trying to figure out why I'm wrong and the time runs out. What's worse is you're kicked off the discussion page when the time runs out too. So on top of failing the practice session, you also don't know why you're wrong.
I've lost my progress lots of times :(. I have clicked "Back", closed the practice tab, clicked on a bookmark in my bookmark bar.
As a temporary solution I suggest finding an addon for your browser which can lock and protect your tab.
For now I found an addon called Tabloc for firefox that prevents closing the tab and opening a link, but it cannot block the "back" option.
btw these are other discussions regarding the same issue:
I would like a restart button or something. It annoys me when i make an amateur mistake such as getting a gender wrong at the first question. I then want to start over again but i have to go back to the home page and then click a few more times before i get to start over.
I definetly agree with what you say about the back button it has happened to me a few times too.