The problem I was seeing wasn't to do with a shared profile, but I have found the same thing happening here on jamroom.net, you can reproduce it like this:
Update a forum post, add this html to the textarea:
<ul><li>asdfa</li></ul>
It validates
Update the forum post again with a line break in the html like this:
<ul>
<li>asdfa</li></ul>
It doesn't validate
Update the forum post again with the line break in after the list item like this:
hypnoscott:
Michael was on skype with me for 45 minutes being incredibly patient and helpful and not ever being rude about the fact that it was not at all his fault.