Answer:
When Mr.White has to decide whether to take back his wish (the one where he wishes his son back to life for his desperate wife) or not and deal with his wife's uncontrollable grief
Step-by-step explanation:
When Mr. White hears the knocking at his door he knew that herbert would not come back the same. So he (supposedly) used his last wish (3rd wish) to wish herbert back to his grave. (or something else it doesn't say in the story)
Hope this helped :)