It shortens an https:// link to http://, and it also shortens invalid links.

Trying to add a js alert I noticed the preview allows arbitrary html to display, but the end result was useless - http://xiv.in/ZW4NL and trying different schemes caused http to be prepended to the url by default.

I would suggest trying to validate the urls, and at least attempting to connect and get a response once before hashing them. This will at least result in less junk in your database (or wherever you're storing them) if you dump invalid links altogether.