Earlier quoted context omitted.
There is a perhaps apocryphal story about the time when Mizuho Securities lost ~$225 million by attempting to sell 610k shares at 1 yen instead of 1 share at 610k yen. When asked whether he intended to fire the person who physically keyed in the order, the relevant department head stated that he did not intend to waste the firm's substantial investment in teaching them to act with due deliberation.
I wish that story ended with the developer of the trading software learning a lesson about user interfaces
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 :)