Say your posting a response in a thread...it would be super cool if a statement flashed when someone else added a response in the thread ahead of you...like gmail does when you are responding to an email and a new email in the thread comes in....
Cheers
SMF does this by default.
If another users posts in the same topic between you viewing the topic and hitting submit on a reply,
Instead of posting, it will redirect to preview with a large notice in RED to inform you that you may wish to re-evaluate/alter your post as someone has posted in it
LOL
Thats never happened for me.
Cheers
In your Profile under Look And Layout Preferences:
[ ] Don't warn on new replies made while posting.
You probably switched it off at some point, if you have, un-tick the box.