For example, after copying one post from some external site to text area this is what I get (toggle view enabled, tested on this forum with Firefox 10 but it was the same on earlier versions as well): hxxp:img19.imageshack.us/img19/149/f4992dc1a4244b5c8ab083c.png [nonactive]
And this is what I get after pressing the preview button: hxxp:img851.imageshack.us/img851/333/2395ebd4ad824ff69a57133.png [nonactive]
Doing the same on my forum after pressing the preview button I'm getting: hxxp:img849.imageshack.us/img849/3885/368fa0eb754b4ab09182c07.png [nonactive] (notice joined paragraphs, and lost heading attribute).
With Google Chrome 17 it gets even worse.
After copying to text area on this forum: hxxp:img195.imageshack.us/img195/964/mirrorby.jpg [nonactive]
And after pressing previewing it: hxxp:img848.imageshack.us/img848/3625/mirror1g.jpg [nonactive]
Pretty much the same on mine forum as well. And I get complains from users, so it's not just me.