I get HTTP 500 and HTML content, what's wrong?

When I got on this page (same with lots of articles on this website) : http://ift.tt/1puhohi

I get an error HTTP 500 (see in the Chrome Dev Tools) AND the article.

Well, I'm a bit lost with this. Do you know why it is designed like this ?

That's a problem for my crawler which is designed to avoid processing HTTP 5xx error responses.