After Mr. White makes his 1st wish on the monkey's paw for 200 pounds and nothing happens, he and his family actually think that the story Sergeant Major Morris told them about the magic contained in the monkey's paw was just a joke.
It could be a coincidence, that Herbert, the White's son, has an accident at work the next day and as another coincidence, the amount of compensation for his death is exactly 200 pounds.
After Mr. White agrees to make the 2nd wish at his wife's urging, for his son Herbert to be alive again, nothing happens, the Whites' go to bed and don't know what to believe. They mention that Herbert has been dead for 10 days, Mrs. White suggests that the cemetery is 2 miles away.
However, some time in the night there is the sound of walking outside their remote cottage, and then someone begins banging on the door. It could be just a coincidence that some unknown traveler has lost his way and is banging on the White's door for help.
It would also be a coincidence that the banging stops just as Mr. White makes his 3rd wish, for his son, to return to the grave where he belongs.
Therefore, it could be just a coincidence, that when Mrs. White opens the door seconds after Mr. White has made his third wish that the lost traveler has walked down the road and can no longer be seen by Mrs. White when she opens the door.