Only applies to some trades of some securities on some exchanges. Even in the most pedestrian cases, NYSE and Nasdaq, you can be up to 20% off after-hours and with portfolio margin this can wipe you out instantly. These rules were created to prevent glitches due to automated trading that might cause significant market meltdowns, not to correct trader's mistakes.

"Harouna Traore, who was taking a class in Paris to become a day trader, used a demonstration version of British brokerage firm Valbury Capital’s platform to learn how to trade equity futures last summer, according to the Financial Times. He opened an account at Valbury with about 20,000 euros, or $23,000.

A short time later, he was at home practicing on what he thought was the demo, racking up more than 1 billion euros of orders in U.S. and European stock futures, and losing more than 1 million euros. Then, Traore came to the shocking realization that the trades were live."

https://www.cnbc.com/2018/06/21/trader-builds-5-billion-posi...