This is actually not true. Previous bubbles were surprising to most everyone, because the general accepted reality was that of a "New Economy." The previous tech bubble did not become widely known as a bubble until it popped.
It's not that it isn't true, it's that it doesn't matter. There are always people screaming bubble - look at the housing market collapse. The thing is, there are always people screaming the opposite. A bubble continues because more people are saying the price should be higher than are saying it should be lower. That continues to build until, suddenly, the tide shifts and then accelerates in the other direction.
