When the Swiss National Bank suddenly let its country's currency rocket on Jan. 15, Pawel Jaworski, a welding engineer from Elblag, Poland, was a loser. Using an online account with Copenhagen-based Saxo Bank A/S, Mr. Jaworski had bet the Swiss franc would fall. He closed his trade with a loss of EUR1,000 ($1,148). "I thought, 'I'm not the winner, but it could be worse,' " said Mr. Jaworski. It was. That evening, Mr. Jaworski, 38 years old, checked his account again. Saxo had changed the price at which his trade was executed. He was out an additional EUR2,000. "This is two or three months' wages; this is money that I ... (full story)