fatal

This word carries a heavy, irreversible weight. When applied to health or accidents, it describes a definitive end, leaving no room for recovery or survival. It is the clinical and absolute version of lethal, often used in official reports or news summaries to signal a final outcome.

In a non-lethal sense, it describes a mistake so catastrophic that it destroys any chance of success. While a mistake might be "bad" or "serious," a fatal mistake is one that ensures the project or plan cannot be salvaged, creating a sense of inevitable doom.