Zou Shiming of China successfully defended his Olympic light flyweight title by edging a controversial contest with Thailand's Kaeo Pongprayoon.
Leading 6-4 after two rounds, Zou came under a sustained assault in the third and was penalised for pushing - adding two points to his opponent's score.
But Pongprayoon was handed the same penalty moments later, allowing top seed Zou a 13-10 triumph.
Pongprayoon fell to his knees and banged the canvas in frustration.
His coach also reacted angrily and lifted his fighter's arm to indicate who he felt was the true winner, while the crowd voiced their displeasure.
Zou, the three-time world champion, became the first boxer to win successive light flyweight golds.