3. James Harden
Former MVP James Harden famously started his career in Oklahoma City, playing next to two other future league MVPs. However, a baffling move by GM Sam Presti saw him trade Harden to the Houston Rockets, essentially ending what could have been a dynasty before it really got started.
Not only that but Harden flourished in Houston, establishing himself as one of the best offensive players in NBA history and even finishing in the top-3 in MVP voting for three straight seasons, including winning it in 2018. Harden later forced his way out of Houston, then out of Brooklyn, and out of Philadelphia, breaking ankles and burning bridges along the way.
Had Oklahoma City not traded him with a season left on his contract, they might have made it back to the NBA Finals and won a championship. Had that happened, then ownership may have been willing to pay the luxury tax to extend him.
They could have kept Harden, Kevin Durant, Russell Westbrook, and Serge Ibaka together for even longer, as the salary cap and luxury tax rocketed soon after he was traded, making their decision to deal him one of the biggest mistakes in NBA history.