Defining "bubble" can be surprisingly difficult and in some cases the illusion of a bubble persists after history proves the speculation or enthusiasm to have been correct despite an earlier collapse in prices. So one interpretation of "Not all bubbles have negative consequences for the economy" is that there are events that are widely perceived to have been bubbles that were not, in fact, widespread misallocations of investment capital.
Defining "bubble" can be surprisingly difficult and in some cases the illusion of a bubble persists after history proves the speculation or enthusiasm to have been correct despite an earlier collapse in prices. So one interpretation of "Not all bubbles have negative consequences for the economy" is that there are events that are widely perceived to have been bubbles that were not, in fact, widespread misallocations of investment capital.