A financial trader monitors data as a television shows euro currency banknotes at the Frankfurt Stock Exchange in Germany.

He thought he was practicing — but the trades and the millions of dollars of windfall were real.

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.

"I could only think of my family," Traore, married with two children, told the FT. "I was stressed."

But instead of stopping, he continued to trade, according to the FT's report, building up a $5 billion position in U.S. stock futures and turning his loss into a profit of about 10 million euro, or $11.6 million.