Happens in Chrome, Opera and Firefox (Not in IE. Safari untested)
Latest stable builds.
I'm using Windows 7 SP1.
Tested on XenForo.com, they should be running 1.1 Beta 4.
Not tested anywhere else.
Test thread: http://xenforo.com/community/threads/this-is-a-weird-bug.22637/#post-282941
As you can see in my test thread, I have formatted some words (made them bold, italic).
When I try to edit those posts, and I put my cursor in front of a formatted word in the begin of a phrase to create a new line, it doesn't work.
It simply doesn't give me a new line.