You do have to watch out with those URL shorteners
Our company sends shortened URLs to the public, usually with 5 character codes, but we did have a one-off request to send some 4 character codes. What could possibly go wrong? It didn't go through the usual process and as such, it wasn't automatically checked against our rather amusing file of expletives. I wasn't going to bother checking manually but then thought better of it at the last moment. Just as well I did as we were about to send one poor guy a URL literally ending in /CUNT.