If you haven’t encountered “smart” pasting, here’s what happens. You copy some text or a URL from your Web browser, paste it into your code or an IM, then find that the site somehow added extra text to it.
I love love love the New Yorker — so well-written and well-edited — and I am horrified to learn (via SVN) that they have installed this atrocity.
What makes this a usability mistake? It behaves in a way that you don’t expect. If you’re going to break expectations, the bar is very high: You must delight people.
This particular trick too often ends up annoying people. See the comments on the SVN post. No delight there that I can see…