875
The one where if you are typing up your comment and then someone else posts a new comment before you post (which also means you haven't read theirs yet). When you click "Post reply" it will have a warning message pop up saying that there was a new reply posted so you can click to view it before posting a message that would need to be edited based on what the new post is.
For example I am typing this message out for the number 875. If someone else posts while I am doing this, my post should be 876 instead. Warning message would be helpful here. Same things goes for that story thread and any other thread really. =d