Electronic trading may be cheap, but leaning on the keyboard can be costly.

A mystery plunge in the value of French 10-year bond futures on July 23rd was triggered by a bank trader at Salomon Brothers in London who accidentally and repeatedly hit the "Instant Sell" button. A wave of 145 separate sell orders sent the price diving on electronic screens.

The disputed trades arose as a result of the prolonged, unintentional and inadvertent operation of the Instant Sell key, said an investigation by computer software firm Cap Gemini and security group Kroll Associates.

Salomon Brothers declined to comment on any losses.