After Bob finally reunites with Jimmy after 20 years apart—and spends 10 minutes walking with him in the rain, telling his old friend all about his life—the man reveals that he is not Jimmy after all, but a plainclothes police officer who pretended to be Jimmy in order to arrest Bob. The following passage—which comes as the two men are walking together—foreshadows this big reveal:
“How has the West treated you, old man?”
“Bully; it has given me everything I asked it for. You’ve changed lots, Jimmy. I never thought you were so tall by two or three inches.”
“Oh, I grew a bit after I was twenty.”
“Doing well in New York, Jimmy?”
“Moderately. I have a position in one of the city departments.”
In having Bob note that “Jimmy” has “changed lots,” including by growing two or three inches, O. Henry foreshadows the fact that this man is not Jimmy. The fact that the man responds “Oh, I grew a bit after I was twenty” likewise foreshadows that the man is a police officer intentionally coercing Bob into believing that he is the old friend Bob has been waiting for. In other words, it is clear that this is not an accidental case of mistaken identity. The fact that “Jimmy” tells Bob that he has “a position in one of the city departments” more directly foreshadows the fact that “Jimmy” is actually an undercover police officer (since police do, in fact, belong to a city department).
Before sharing Bob’s backstory as a criminal in Chicago, O. Henry subtly foreshadows this fact in his description of Bob’s appearance:
The man in the doorway struck a match and lit his cigar. The light showed a pale, square-jawed face with keen eyes, and a little white scar near his right eyebrow. His scarfpin was a large diamond, oddly set.
There are several different clues in this passage that hint at Bob’s criminal status. First, O. Henry’s description of Bob’s “little white scar near his right eyebrow” signals that Bob has been in some sort of fistfight or physical altercation that left him scarred. While people can have scars from things other than fistfights (and not all people who get into fistfights are criminals), in a story as short as this one, every word matters, and O. Henry is clearly hoping to signal something by highlighting this facial scar.
The other descriptions of Bob’s face—as “pale” and “with keen eyes”—also subtly signal that something is amiss with this man. Bob’s paleness can be read as a sign of his misdeeds (and likely ongoing run from the law) making him unwell, and his “keen eyes” hint at his need to be on his toes and self-possessed, given that he is on the run from the law.
Finally, the description of the “oddly set” and “large” diamond on his scarfpin (a common accessory for men at the time) foreshadows the fact that Bob is a criminal with misbegotten wealth—the oddly set nature of it signals the immoral way that Bob amassed his wealth and the large size signals the scale of his crimes.