The short story "After Twenty Years" by O. Henry does have a tragic ending as two close friends who have vowed to reunite after twenty years, find themselves too changed to remain friends.

Both grew up in New York and were as close as brothers. Bob, the younger, went out West to find his fortune, and he appears very successful as he has a watch "covered with small jewels." Bob waits for his friend, Jimmy Wells, as a cop strolls by, checking doors to see if they're secure. Bob stands in the doorway and tells the cop he's waiting for his friend of twenty years to meet him. The cop wishes him luck and walks away.

Unfortunately, Bob has turned to crime for his livelihood, so the cop, who turns out to be Jimmy Wells, has to make a moral decision. Does he arrest his closest friend who has traveled a thousand miles to keep his word to meet him or does he let him go? As a law enforcement officer, Jimmy makes the choice to have Bob arrested.

This situation is tragic as it weighs the value of friendship and loyalty to the demands of a job as a policeman. Jimmy chooses loyalty to his job over loyalty to his friendship, sending another cop with a note from him to arrest Bob. Being forced to make this decision gives the story the tragic twist.

To me, this ending is tragic because it shows us how two people who used to be best friends have grown apart.  And it is tragic because it forces one of the two best friends to actually take away the other friend's freedom.

If you are Jimmy Wells, walking down the street, you are hoping that you are going to see your friend again -- someone who was like a brother to you.  But then you see him and you realize he is a criminal, a kind of person you have sworn to fight against.  This would be incredibly sad -- to find that your best friend has become a kind of person that you hate.

So the story is tragic because it builds up these hopes and expectations on our part.  We hope for and expect a happy reunion between the two men and all we end up with is Jimmy having to have Bob arrested.

How does the meeting in "After Twenty Years" turn from happy to tragic?

In this story, the two men (Jimmy Wells and his friend Bob) are supposed to be meeting again after 20 years.  I am not really sure that the ending is tragic, but it is a surprise.

As Bob waits for Jimmy at the place where they were supposed to meet, he is accosted by a police officer.  The two of them talk for a while and Bob tells the officer what he is doing there.  Then the officer leaves.

Later, a second man comes and starts to talk to Bob.  At first Bob  thinks it is Jimmy, but soon he realizes it is not.  As it turns out, Jimmy was the first police officer.  This second man is another officer sent by Jimmy.  Jimmy sends a note saying he recognized Bob as a wanted criminal but didn't want to arrest him himself because they had been friends.