It's already happened, multiple times.
Mind you, so have people died in the process of boxing, playing football, playing baseball, playing basketball, etc.
And the bigger question ultimately for the sport, is if/when someone may die in the UFC. That's really the true "face" of the sport and is the best equivalent to the major "professional" leagues of other sports.
The reality is, that many of these severe injuries or deaths have occurred in minor and/or local promotions that are more lax in terms of reffing and allow things to go on far to long (the death recently in the UK is a great example of this). For all the people who bitch about the UFC stopping fights early sometimes, it goes a long way in reducing the chance for severe injury or death due to continued punishment.
That leaves basically, at worst, a death due to a freak accident. Which, ultimately, can occur in any sport.