Ronda Rousey
Rousey was a force of nature during her early UFC career. Her opponents rarely made it out of the first minute let alone the first round. The former Olympic judoka arm barred her way to victory and became one of the first true superstars in MMA history (via Time).
Then she fought Holly Holm in a career-defining performance. The former kickboxing champion dissected Rousey on the feet in round one. Rousey went back to her corner for advice during the end of the round but her coach told her to keep boxing. It was a terrible error of judgment because Rousey suffered a vicious knockout and lost her title in a career-defining lapse.