SAN QUENTIN, Calif.—It was an off day during the NBA Finals, and the general manager of the Golden State Warriors was in prison.

Last weekend, as Golden State’s players and coaches arrived at their arena for a light practice, Bob Myers and a team of Warriors executives, former college players and pickup basketball regulars surrendered their cell phones and checked themselves into San Quentin State Prison. The gates of California’s oldest correctional institution locked behind them.

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