Stick around long enough and there will be a time when you're not 100%. Accidents happen outside the gym more often than inside, and it's important to keep training what you can when they do. If you break a wrist, there are many lower body exercises and one arm exercises that you can continue to work on. If you have surgery on both hips, that gives you an opportunity to work on upper body strength. CrossFit has a pretty broad range of possible exercises, and I bet that no matter what is going on to prevent you from doing one type of movement, there are others you can work on until you're better.
Moral of the story: don't let an injury keep you out of the gym, whether it's a paper cut or a cast!