The most egregious case of this comes from President Warren Harding. Shortly after his death, his widow destroyed all his personal notes, memoirs and documents. Since then, biographers have had a field day since they felt they had free reign to write whatever they want knowing that they wouldn't be fact checked since evidence was destroyed.
This is why when someone starts speaking after it's been established that nobody knows the answer, I tune out. Chances are they're just exploiting the circumstances.