Fatalities? Not that I'm aware of or published in our main journals.
Serious injuries? Yes. These are invariably due to improper technique. If the fat is harvested under too much pressure, or if it is mishandled during processing, or if it is injected too much at a time you can have problems. The main problems are fat necrosis (the fat cells die) and infection. Both can result in disfiguring scars.
When properly done Fat grafting to the buttocks is extremely reliable and safe. I have not had a redo in over 4 years and the only complication has been an infection in the belly where the fat was taken from.
You can read more about the specific steps involved in fat grafting in the link below so you can ask the right questions when you go in a consult.
