It seems like every asset bubble has a famous anecdote of someone claiming, right at the top, that a crash is impossible. In the great Bitcoin bubble of late 2017, the honor goes to John McAfee, founder of computer security company McAfee LLC, and passionate cryptocurrency evangelist. On Dec 7, 2017, he wrote:

Bitcoin now at $16,600.00. Those of you in the old school who believe this is a bubble simply have not understood the new mathematics of the Blockchain, or you did not cared enough to try. Bubbles are mathematically impossible in this new paradigm. So are corrections and all else

A few days later, Bitcoin’s price briefly peaked at $19,511, before it began an epic plunge that would see the original cryptocurrency lose approximately 82 percent of that value as of the writing of this column:
https://www.bloomberg.com/opinion/articles/2018-12-11/yep-bitcoin-was-a-bubble-a...

Of course with every bubble we will also here the retrospective anecdotes of how people saw it coming and sold the day before the crash. Just like gamblers only ever talk about their wins