Please add a pop up that says something similar if you try to leave the page while writing a comment.
I'd imagine you finished typing a multi-paragraph comment and accidentally Alt+F4'd?
Or just navigated via the "back" of the browser. There are hotkeys for that or mouse context menu misclick can do it as well.
I'd also like to see an alert for when you're typing a post on a thread that has been deleted since you started your post. Perhaps this is a topic for another thread, though.