I've seen this as well. It seems to happen when your selecting between two divs. Below is an example of what I'm talking about:
Typed Text (The yellow are suppose to be the higlighted text):
Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh euismod tincidunt ut laoreet dolore magna aliquam erat volutpat.
Ut wisi enim ad minim veniam, quis nostrud exerci tation ullamcorper suscipit lobortis nisl ut aliquip ex ea commodo consequat.
Higlight HTML:
<div>Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh euismod tincidunt ut laoreet dolore magna aliquam erat volutpat. </div>
<div>Ut wisi enim ad minim veniam, quis nostrud exerci tation ullamcorper suscipit lobortis nisl ut aliquip ex ea commodo consequat.</div>
Under this circumstance i've had the same problem. I've updated to the new 5.1 version is happens a lot less, but still happens once in a while.