Some browsers use unofficial/non-standard URLs for various purposes. For instance, in Firefox, there are the about:, about:config, and about:buildconfig URLs. The trouble is, some websites (e.g. reddit) don't recognise these as clickable links. So the natural thing to do would be to use a tinyurl service as a go-in-between. However, if I submit about:buildconfig to tinyarro.ws, it treats it as if I had submitted http://about:buildconfig, and that's incorrect, so it doesn't work. A little digging revealed that many other URI schemes, both official and unofficial ones, don't work either. Not even mailto:email@domain.tld works. Now you could of course say that you only want to support the http: scheme, but you already do support ftp: and gopher:, so why those and not others such as mailto: (official) and about: (unofficial)? You may even consider allowing just about anything as long as it can't harm your site.