SAN FRANCISCO — They were once colleagues at Korn/Ferry International, the global executive search firm at the center of a computer-crime trial that began this week.

But for close to a decade, David Nosal, the former executive on trial, has been a thorn in the side of Korn/Ferry’s longtime general counsel Peter Dunn, making waves as he exited the firm in 2004 and then embroiling the company in years of expensive litigation.