Extra string at end of product info url

I noticed today that at one of the sites I'm working at there is an extra string starting with #. after the product_id when viewing the product info on an item in the store.
I don't think this string has been there before and I can't find out what is causing this extra part to appear on just the product info pages (as far as I know this string only turns up on those pages).

The site is www4. carmo.nl
At the moment I'm still working on it so some stuff may not work exactly as intended.

Does anyone know what may cause this to happen and how to turn this off?

Okay, I think I just found the cause of the extra string. I had looked for several hours already before posting my question and several minutes after posting it I think I found the solution.

After removing the addthis part of the page the extra string no longer turned up. The addthis part was something we were still thinking of whether it should be included or not, so removing it probably won't be an issue for this site.

Yes, the AddThis script adds that junk.

AddThis is also prone to slowing down the page-load time - and as PageSpeed is now a central facet of Google's evaluation of a site's UX, I would not install anything external that slows page-loading speeds.

Also - when we had this on some of our sites - fewer than 1 in 10,000 visitors ever bothered to use it. They have come to shop, and to checkout as quickly and as easily as possible.

There are other ways of leveraging social networks for wider reach. AddThis is a complete waste of time. We dumped it some time ago.