Today I clicked Reply on a post, edited to a subset of the quoted text, then entered my reply. While I was composing my reply the warning banner about new posts to the thread came up. I clicked on the "view them" link. Then when I clicked the Post Reply button the new post box was emptied but my reply was not added to the thread. It was just lost, so I had to recreate it. This was the second time I've had a reply disappear, but the first time was a while back and I don't remember if I viewed new posts in that instance.