The work of Yale economist/historian William Goetzman in his paper, "Bubble Investing: Learning from History" analyzes the behavior of large asset price booms to determine whether all extraordinary increases lead to large crashes. His conclusion is that bubbles don't always bust. This is odd given the view that whenever there is a market with accelerating prices, say a doubling in price, there is talk of a bubble. There is often as much talk of the potential price reversal as discussion for the price increases, yet the numbers tell us something different.
What makes this important is that the fear of booms and the potential for a bubble may be misplaced. There is a higher probability for a bubble when there is a boom, but the conditional frequencies are such that the idea that all booms should be avoided because there will be a crash may be misplaced. What goes up quickly does not have to reverse quickly. Concern for accelerated price increases, yes. Fear from market doubling as a basis for a bust, no.