John Gruber at Daring Fireball posted this interesting article on the growing practice of websites intercepting your attempts to copy text from their pages. They are actually modifying the contents of your clipboard and tracking the fact that you have clipped the information.
The referenced cases seem to be doing it for marketing and informational purposes, but there are many ways this could be used in more aggressive ways.
Imagine a site with sample code which (when copied) inserted some damaging code in to the middle of a large block.
I am worried that this capability exists at all within browsers. It seems like a major security vulnerability to me.