I made 17.5k doing this by dumb luck.

I was trading crypto back in 2016-2017. Making a few thousand here and there. Mostly just from time in the market during the run-up of ethereum. The whole thing drove me crazy because I thought it was so stupid, like gambling. So in 2017 I withdrew my several thousand dollars and put in a trade on coinbase pro for 5 etherum at $100. (I had left $500 to do this, the most I was willing to risk at the time) On 2018-11-25 this trade was executed in a flash crash.

I had totally forgotten about this trade and when I was going to buy a house in 2021 I was looking for any change in the seat cushions thinking I may have left a small amount in these accounts. Well imagine my shock when the amount was just short of $18,000.

Glad it worked out for you! Thanks for sharing.

HFT algos are going to make quick work of the opportunity well before your broker can even think about executing your limit order.

You want your GTC order parked on-exchange, not sitting on a broker's private book. Find a better (more expensive) broker.

I thought the real market also has circuit breakers in place. If the market falls X% too quickly, trading is halted for some amount of time.

That applies to the whole market, not necessarily a single stock.

It definitely can apply to single stocks, depending on the exchange. For example I believe NYSE has single-stock circuit breakers that kick in around 15%, then subsequent further breakers once trading resumes.

If you aren’t paying attention you just might end up trading on a real crash too.

Which I guess is why it has to be so low you don't care.

Yes we call this a stink bid, and it’s a valid strategy. Keep one open just in case. Most exchanges have slippage protection in place but some are better at it than others.

I believe, the traditional exchanges either won't fill outrageous trades or would undo them if the counterparty can prove it was an honest mistake.