BJ Penn
Penn’s partying derailed his MMA career. He was one of the best fighters in the world as a rare two-division UFC champion. The Hawaiian fan-favorite produced exciting matches and many great rivalries. But then he went on a dismal losing streak that affected his legacy in the sport.

He was partying too hard and had multiple embarrassing street fights. One of these saw him give a man a free shot to hit him on the chin. He woke up after discovering that his rival knocked him out. This also affected his future because his erratic behavior made it too risky to give him TV work like other veteran fighters (via GiveMeSport).