What testimony has been given in court by the old man who lives in the same building as the suspect in Twelve Angry Men by Reginald Rose?

The jurors examine and question the old man's testimony in scene 6. During the trial, the old man, who lives downstairs from the scene of the crime, testified that he heard arguing coming from the apartment above him, including a threat of murder. He then testified that he heard the sound of someone falling down. Alarmed, he ran to the window down the hall and saw the boy running out of the building. He then called the police.
This testimony makes the old man the prosecution's star witness. It certainly seems like enough information to convict the boy of the murder. However, Juror 8 wonders if the old man's story is accurate. Apparently, the witness was in a feeble state, having suffered a stroke some time earlier. He entered the courtroom walking slowly with two canes. Juror 8 does not think that the old man could have had enough time to make it to the window to see what he claims he saw. To illustrate this, he makes a diagram of the apartment on the floor. Moving at the speed of the old man, Juror 8 takes over 40 seconds to cover a distance that would have needed to have been covered in 15 seconds. Furthermore, the timing of the passing train means that it would have been near impossible for the witness to have heard what he claims he heard.
This presents a major flaw in the witness's testimony. Juror 9 suggests that the old man, feeling discarded by society, might have been inventing facts for attention. He tells the others that

A man like this needs to be recognized. To be questioned, and listened to, and quoted just once. This is very important.

All this introduces reasonable doubt into the old man's testimony. Indeed, it is this weakness in the testimony that finally convinces several jurors to acquit.

After the jurors cloister themselves in the jury room and they take a vote, Juror No. 8 is the only one who votes "Not Guilty." When Juror No. 12 suggests that they convince him, No. 3, a very opinionated and intolerant man, goes over the facts. First, he refers to the neighbor's testimony.
The old man, who lives on the second floor under the room where the murder occurred, has testified that he heard noises that were quite loud coming from the upstairs apartment. To him, there seemed to be an argument going on; then, he heard the boy say to his father, "I'm gonna kill you." Right after this, he heard someone falling; so, he ran to his door to look out, and he saw the suspect running down the stairs and out of the house. After witnessing the boy's running away, the old man phoned the police. When the police arrived, the father was found with a knife in his chest.
This testimony seems rather damning, but it is the perseverance of Juror No. 8 in examining and re-examining the testimony and other facts of the case which lead them to feel that there is reasonable doubt about the son's being the one who stabbed his father.