A terrifying thought:

John McCain was born on an overseas US military base, not US soil.  Because of this, substantial uncertainty exists about whether the Constitution permits him to assume the office of the presidency.

IF the McCain-Palin ticket wins the election, and IF someone were to challenge the legitimacy of McCain's presidential qualifications, and IF the Supreme Court were to rule that he is excluded by the Constitution from serving as President, THEN Sarah Palin would become the next President.

We wouldn't even have to wait for the high actuarial risk that McCain won't live through a complete term.  The Supreme Court could make Sarah Palin the next US President on Day 1.