You're right, my "mess up once, get fired", I really mean if a flight crashes and the FAA could have done something, they'll get political flak. High level political appointees may need to resign. Whereas if an agency does nothing for a term, the political leaders get to stay.

For routine FTEs, as a sibling post mentioned, you can probably mess up in a lot of ways (short of murder or being really non-PC) and still keep your job.