I've lived through 3 and read up on at least 10 more.
Many people know it's a bubble. You see lots of HN comments that it's a bubble. We certainly did in 2007.
In a bubble, the predominant notion is not "I wonder if it's a bubble", but rather "It's a bubble, I hope I can make my money before it pops". A perfect example of this was the Tulip craze in the Netherlands. Nobody actually believed tulip bulbs were worth that much, they just wanted to find a bigger sucker to make a profit, if you read accounts from the time.
What is the false positive rate? Broken clock...
Bears predicted 11 of the last 3 recessions or something.
This. Ans that's why we even manage to reproduce bubbles in a very simple simulated environnement among finance students who know exactly how overvaluated their assets are (I can't find the paper right now though…)