Notice that new post has been added to thread, like gmail, would be very cool!

[weightman]

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

[karlbenson]

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

[weightman]

LOL

Thats never happened for me.

Cheers

[Zetan]

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.