Bostrom notes that we haven’t drawn a black ball so far, but “The reason is not that we have been particularly careful or wise in our technology policy. We have just been lucky.… Our civilization has a considerable ability to pick up balls, but no ability to put them back into the urn. We can invent but we cannot un-invent. Our strategy is to hope that there is no black ball.”
The idea of a "black ball" implies that you can predict that such an event will occur. Sometimes what you have is a "black swan" event that you can't predict.
