In "After Twenty Years," Jimmy makes a decision early in the story to conceal his identity when Bob strikes the match to light his cigar. How does that decision leads to the surprise at the end of the story?

O'Henry is well-known for his surprise endings. In "After Twenty Years," Jimmy makes a decision to conceal his identity as Bob's long-lost friend and as a police officer. He keeps the appointment that the two men had arranged years earlier, but as he walks up to Bob, he recognizes the face of a wanted criminal. Jimmy cannot bring himself to arrest his old friend, so after chatting to make sure it really is Bob, Jimmy walks away and asks another officer to arrest Bob.
Jimmy's decision directly affects the surprise ending. The audience is just as surprised as Bob is when he reads Jimmy's note: "I was at the appointed place on time . . . I went around and got a plain clothes man to do the job." At first, it seems unexpected that Bob's old friend, the man he describes as "a kind of plodder" yet "the truest stanchest old chap in the world" would turn in his buddy. However, O'Henry does give us clues. From the start of the story, Jimmy is described as "watchful" as he moves "impressively" down the street, making sure that the streets are safe. We get a sense of security as he tries doors to be certain they are locked and addresses the man who is standing idly in the doorway. Securing the safety of the neighborhood seems to come naturally to this "guardian of the peace" as he walks away from Bob, still "trying doors." Jimmy must have been shocked to see that his old friend was a wanted criminal but, always the professional, he remains cool and does not show his emotions. He remains a true law enforcer as he chooses truth and morality over friendship. When Bob thinks back on this situation, he will certainly realize that Jimmy could not have acted in any other way.