Don't ask me how I know this, even if I explained myself to minute detail I wouldn't be able to prove it to you anyway. I'm revealing this to the community just to clear my own conscience.
tl;dr; Johnny McCript (fake name for obvious reasons) deviced a way to mine Bitcoin faster without 200 tonnes of hardware and terawatts of electricity.
He was sitting in his bedroom hacking some code and found a way to craft Bitcoin blocks in a way that he can deliberately solve a block whenever he wants to.
I was there when he ran his first successful test.
McCript lit up a Camel cigarette and laid back onto his chair, staring at his monitor in awe. "WTF have I done" he mumbled.
I asked what happened and he said he'd done something big, really big.
He went into lots of details, but this is what I can remember: he'd taken some TX's from mempool, assembled a block, solved it instantly and broadcast it into the network. The block reward popped up in his balance shortly after.
12.5 Bitcoin - just like that.
He opened up Reddit, Twitter and bitcoin-dev group and immediately started typing a description of his exploit. He said this would make him famous. I instinctively asked why he'd put himself at risk for nothing.
Suddenly he pushed the keyboard away, blank expression on his face.
He stared at me blankly and seemed to be asking himself rhetorical questions. He knew I knew nothing about this business, so I couldn't possibly answer any of that stuff. But he'd listened to my quip about being at risk.
Why destroy 215 billion dollars in value just like that?
Why not go along with the game?
What if others had found the same thing before him and also went along?
If he revealed this would people want to kill him? Would he be in danger?
So instead of wrecking Bitcoin, Johnny decides to use his discovery to make tons of $. Every now and then he creates a few blocks, makes some $$$ and lives the good life.
Last time I met him he explained this bit to me. By solving blocks, the network statistics estimators translate this to hashrate. So Johnny was solving blocks with some simple hardware but the network estimated his hashrate to be some monster mining operation. But it's just him, instantly solving blocks for cash.
The hashrate flash crash happened earlier this week when Johnny's electricity went offline for about 16 hours due to a major storm that hit his region. His garage was flooded 3 feet up. Worst day was September 23.
His systems are back up and everything's normal again.