how did juror 8 discredited the knife

This is the first moment when 8th Juror really makes a crack into the case of the prosecutor. He proves that the knife claimed to be "one-of-a-kind" that the defendant owned wasn't as original as they may have been led to believe. Negating the legitimacy of evidence in this way lends itself to the creation of reasonable doubt, which, if it is exist, is grounds for a verdict of 'not guilty.'