The main situational irony in "After Twenty Years" involves the fact that Bob is waiting for Jimmy and doesn't realize he is actually talking to him. Then when Jimmy leaves, Bob thinks Jimmy didn't show up, but he continues to wait. It is ironic that he doesn't realize that he is waiting to get arrested. Jimmy makes a point of asking him if he is going to be there much longer.
“I'll be on my way. Hope your friend comes around all right. Going to call time on him sharp?”
“I should say not!” said the other. “I'll give him half an hour at least. If Jimmy is alive on earth he'll be here by that time. So long, officer.”
And finally when the plainclothesman posing as Jimmy appears, Bob thinks this is Jimmy arriving to meet him as promised twenty years ago. Bob appears to be a successful confidence man, but he is fooled by both Jimmy and the plainclothes detective. It is also ironic that Bob has traveled a thousand miles only to get himself arrested. A further example of situational irony might be that Jimmy expects to meet his old friend and to go someplace where they can have a long talk about old times. But when Bob lights his cigar, Jimmy realizes that this is the man the Chicago police want apprehended and that he is obligated to arrest him or at least to have him arrested by somebody else. It is ironic that Jimmy was looking forward to a pleasant meeting and ends up feeling disappointed and a little bit guilty. Bob feels sure that the cop is a cop and not Jimmy. He also feels sure the plainclothesman is Jimmy and not a cop. In both cases he is wrong. It is ironic that he strolls up the street arm in arm with the man who is arresting him and ironic that he tells him all about his life of crime during the past twenty years. Bob is a great talker. He hardly allows the uniformed cop or the plainclothes cop to get a word in. Then when they get to the lighted drug store the whole situational mix-up is made clear to the reader. The plainclothes detective tells Bob:
“You've been under arrest for ten minutes, ‘Silky’ Bob."
The fact that the scene is suddenly "brilliant with electric lights" is appropriate to the enlightenment that is going to clarify the train of events for both 'Silky' Bob and the reader.