Is any Supreme Court nomination invalid?
James Lindgren, on the Volokh Conspiracy, reports an interesting point.
By law, there are nine Justices on the Supreme Court, and the President can nominate, and the Senate confirm, a successor when a Justice resigns or dies.
But O'Connor's resignation said it would become effective when her successor was confirmed. So her resignation won't take effect until the successor is confirmed. But until it takes effect, there is no opening, and no one can be nominated or confirmed. Catch-22, the best catch there is!
In practice, of course, the problem will be ignored (as it apparently was the last time it arose, when CJ Burger resigned the day his successor, CJ Rehnquist, was sworn in).