Why was there 'reasonable doubt' about: the old man's testimony; the woman's testimony; and the knife?

4th Juror presents the death weapon, the “only one of its kind;” 8th Juror surprises the others by presenting an identical knife he had purchased in a pawn shop two blocks from where the boy lived a few nights prior, shattering the claim that the knife was unique and identifiable.

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8th Juror presents several hypothetical situations, based on the father’s criminal background, that could have gotten him killed that evening. Next, he attempts to discount the testimony by the old man living downstairs by deducing that, because of the sound made by the elevated train passing by, there’s no way he could have heard with certainty screaming and a body hitting the floor. 9th Juror identifies with the poor old man, believing that he might just be trying to feel important. 8th Juror concludes by saying that even if he did hear him say, “I’m gonna kill you,” that very well could be taken out of context as just a figure of speech.

After a brief argument, 8th Juror brings into question whether or not the downstairs neighbor, an old man who had suffered a stroke and could only walk slowly, could have gotten to the door to see the boy run down the stairs in fifteen seconds, as he had testified. 8th Juror recreates the floor plan of the apartment, while 2nd Juror times him, and they conclude that he would not have been able to reach his door in fifteen seconds.

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4th Juror begins a discussion by reintroducing what he considers to be the most compelling piece of evidence, the testimony of the woman across the street, who claims to have heard a scream and then to have seen him stab his father through the windows of the elevated train passing by. While he is speaking, he rubs his nose where his spectacles have made indentations. This causes 9th Juror to realize that the woman also had those same marks on her nose and must have worn glasses, despite the fact that she didn’t wear them in court, presumably for her own vanity. This causes all of the jurors to question the eyesight of the woman, who may have witnessed the murder without her glasses. Based on this, 4th Juror changes his vote. 10th Juror gives up and also changes his vote to “not guilty.”