Stupidity: There is no way a fighter with a brain would risk serious injury to themselves before a fight by continually punching a frozen rack of meat in a freezer slaughterhouse. There is nothing to gain by doing this, and it would only result in both fists being broken.
Suggested correction: In fact, Sylvester Stallone permanently damaged his knuckles filming the two scenes. He obviously did not realise how dangerous punching a side of beef would be, and it isn't out of the question that the character Rocky Balboa wouldn't realise this either.
It shouldn't take a rocket scientist to know that if you punch or hit anything with a bare fist against anything solid, rock hard, you will damage yourself badly.
Absolutely, it shouldn't, but that is not what the "Stupidity" category is for. It is unintelligent for someone to do this, but that is not a movie mistake. Sylvester Stallone really did punch that beef and really did injure himself, so it is not something that is out of the question for a character in a movie to do.
No, but it is out of the question for a man with painfully damaged knuckles to go fifteen rounds with a world heavyweight champion boxer. In fact, Rocky would probably have failed the compulsory pre-fight physical and would have been ruled unfit to fight.
Where in the movie is Rocky experiencing "painfully damaged knuckles"? With your logic, the fight would have most likely been called long before the 15th round, due to the very unrealistic and improbable punishment the 2 boxers were inflicting on each other.