I suppose the most famous example was Michael Heseltine's declaration that he "could not foresee the circumstances" in which he would challenge Margaret Thatcher, allowing him to launch just such a challenge when the previously unforseeable circumstances actually came about.
So I applaud David Miliband's decision this weekend to deliver an unequivocal statement that he will not challenge Gordon Brown for the Labour leadership.
It's no less than confirmation of what he has always said, but it represents a victory for political plain speaking that Brown himself would do well to take note of. More on this theme later....
That apart, I have no doubt that Miliband has made the right decision, and I explained why in my weekend column and acompanying podcast.
If you can't be bothered to read or listen to it all the way through, a potted summary might read:
Today's Guardian speculates that either one of Reid or Clarke will still stand, and I share that judgement. Reid v Brown in particular would be a good contest between two men of genuine Prime Ministerial calibre. But neither he nor Clarke can win.
All of which suggests that it's all over bar the shouting.