This is a good point--I remember even the online game "Runescape" had a warning that would pop up if you tried to sell an item worth X for a very low price.

It would seem to me (somebody who knows squat about stock trading) that a warning screen would be much more beneficial for stock trading software than it is for an online video game.

You'd think that, but then what happens is users get used to the warnings coming up and consciously/unconsciously adjust to click by it without reading.

I'd imagine in the case of the stock trader, where quick reaction is valued, he'd have quickly entered whatever key combo is necessary to dismiss it the warning and made the same mistake.

Only if the warnings come up too often. I imagine it should be quite possible to define criteria that discriminate selling shares at a price of 0.0000016 yen from normal prices, by comparing with the market price, or the range of prices ever seen by the software, for example.

Alarm fatigue is an important design consideration, but not a reason to not put warnings at all (given that this isn't amenable to an "Undo" button, which is preferable when possible).

Alarm fatigue is indeed an issue -- but if the trader is repeatedly trying to sell for way below market price that often, then perhaps there is a much bigger problem...with the trader.

It ended up with order sanity checks being implemented for all manual trading systems at Mizuho. Things like blocking an order if it's more than 10% away from the market mid, etc.

You shouldn't fix an issue like this with UI validation alone - it needs to go between the component that creates orders and the stock exchange, so that it also protects against software bugs. For instance, a possible bug is a developer multiplying the order size by the lot size in the backend, when it has already been multiplied in the frontend, causing huge orders to be sent. Sanity checks can catch this.

Automated trading systems have traditionally been under a lot of scrutiny, and nobody in their right mind would run one without sanity checks and a kill switch. That incident taught Mizuho that manual trading can, in fact, also be quite dangerous :)