Boos rang out at a UFC Fight Night in Rio Rancho, New Mexico, as Kazula Vargas was disqualified for an illegal knee to the head of Brok Weaver.
Weaver was sat on the canvas up against the side of the Octagon during the fight when Vargas suffered an inexplicable brain explosion.
You're not allowed to knee an opponent in the head when they're grounded in the UFC but Vargas lost the plot and unleashed one anyway, knocking Weaver out.
Vargas continued to try and punch Weaver after the brutal blow but the referee rushed in and physically dragged the Mexican away before doctors arrived to assess the 28-year-old.
The ref could be heard telling the judges "it's a disqualification" and that was confirmed soon after, handing Weaver his first win in the UFC — but not the way he would have wanted it.
Weaver shook his head as his hand was raised in the post-fight ceremony while fans voiced their disapproval. In a classy display, the American offered words of support to Vargas, who looked shattered at making such an obvious mistake.
In commentary, Daniel Cormier said: "That's as illegal as it gets.
"That's as illegal a knee as I've seen in a long time.
"I don't understand what Kazula Vargas was thinking there, throwing that knee."