Jordan uses the piece of gum found at the phone booth to place a suspect there. But she does not use the piece of gum found in the back of the cab which is the murder scene.
The license plate numbers on Garret's corpse should be mirror image.
The detective says that the cab driver's wife works in a greasy spoon in Quincy. He mispronounces it, saying "Quin-sey" instead of "Quin-zey," as the town in known there.