Any combat sports decision has the potential to be overturned. Some reason for this may include counting errors, misdeclaration, and retroactive disqualification due to rule violations. There has never been a unanimous decision overturned.

Kicking/Kneeing A Downed Opponent In The Head\n\n Kicking or kneeing the head of a downed opponent hasn't been allowed in the UFC since official rules were implemented and is an illegal in the UFC. If a fighter has one or both knees on the ground, they are considered a downed opponent.