From the sacred sex site comes the below tale... I'm not entirely familiar with Padmasambhava, but why exactly does the lama die? Because he practiced Sexual Magic? Or because he did it wrong? 

Dalai Lama: Fate of the 12th

...the Twelfth Dalai Lama had once experienced a vision of Padmasambhava, in which the great guru spoke the following words of prophecy:

O Trinley Gyatso, supreme lord of truth,
You who are my own mind emanation,
If you accept the practice of secret mantra by
Relying upon a Knowledge Lady [jnanamudra],
Tibetan Buddhism will thrive
And the Dharma Protectors will act.
But if you do not rely upon
The siddhi of karmamudra [sexual yoga],
You will soon die.

The Twelfth Dalai Lama died by the age of twenty.