The forums automatically log you out after a set period of time independent of "keep me logged in" setting, and when they log you out, it doesn't reset the UI to get rid of the "Reply" option. If you try to post a reply after being invisibly logged out, it still lets you open the "Reply" box and write a reply, but it does not save the post automatically and when you enter the post, it closes the reply box and eats your post.