The phrase "fatal consequences" means that something has happened that leads to very serious, severe, or deadly outcomes. It means that the result of a certain action or event has had significant negative impact or caused great harm. Full definition
Another operator switched it on while the repair was under way with fatal consequences. (thompsonstradeunion.law)
Damage to the mitochondria thus has fatal consequences for cells. (sciencedaily.com)
This is a special concern when running a hub motor on the front, because if your fork snaps, it can have potentially fatal consequences (think face plant onto the concrete). (treehugger.com)