Alongside the fancy knife that was apparently used as the murder weapon, the eyewitness testimony of a neighbor is the most damning piece of evidence against the defendant in 12 Angry Men. After all, she claims that she actually saw the defendant kill his father.
But this lady's testimony comes into question when one of the jurors remembers that she had marks on her nose from wearing glasses but didn't wear her glasses to court. He explains this by saying that the woman probably wanted to look younger, adding, "No glasses. Well, women do that. See if you can get a mental picture of her."
So now we know that the woman didn't wear her eyeglasses to court. It's safe to assume that she also doesn't wear eyeglasses while sleeping, as one juror says, "No one wears eyeglasses to bed." But according to the woman's story, she sat up out of bed and immediately looked out the window to see the boy killing his father from across the street—and through the windows of a passing train, no less.
Even the cold and rational Juror #4 is swayed by this new evidence, saying, "Strange, but I didn't think about it before." So in this case, the glasses represent just one more way that the court has bent the truth in order to get the Guilty verdict it wants.