Scrummerfall. n. The practice of combining Scrum and Waterfall so as to ensure failure at a much faster rate than you had with Waterfall alone.
I regularly talk to people who claim to be "adopting agile methods", but it's clear than what they've adopted is Scrum (a good start) in a vacuum without any appropriate agile software development process. Remember, Scrum is about project management. It doesn't tell you how to build software.
The worst case scenario, in my experience, is embedding Waterfall inside of Scrum. This often manifests in what I call the One-Two-One pattern: one week of design, two weeks of coding, one week of test and integration. I've yet to see a team that was long term successful with such a system, especially if they are strongly rooted in historical Waterfall. As often as not, they will abandon Scrum after just a few sprints, claiming that it failed to provide anything but pain. Worse, that's often the extent of their foray into agile. They "tried that agile stuff" and failed, so they're sour on it.
Successful agile projects require far more discipline than any other methodology I've ever used. Doing XP right, all the time, is very hard work. Aside from making sure you adopt an agile development process in addition to Scrum, the best piece of advice I can give for success is: hire an agile coach. Having someone who knows how to successfully run agile projects is invaluable; having them sit in the room with you and provide real-time guidance is an incredibly effective experience.