Madame Defarge’s accidental death is an example of situational irony. At the beginning of the novel, Madame Defarge believes that she is an agent of fate. Speaking to her husband about the revolution, she says, “although it is a long time on the road, it is on the road and coming […] Nothing that we do is done in vain."
However, the actual circumstances of Madame Defarge’s death contrast her belief that nothing is in vain. Near the end of the novel, she mistakenly shoots herself during an altercation with Lucie’s servant, Miss Pross. In a letter to writer John Forster, Dickens explained that an accidental death was the only fitting punishment for Madame Defarge, since she would have been happy to die in the street, fighting for her cause.
Manette’s unintentional condemnation of Darnay is an example of situational irony. The words Manette wrote about Darnay’s ancestors while he was in prison are read to the court:
I believe that the mark of the red cross is fatal to them, and that they have no part in His mercies. And them and their descendants, to the last of their race, I Alexander Manette, unhappy prisoner, do this last night of the year 1767, in my unbearable agony, denounce to the times when all these things shall be answered for. I denounce them to Heaven and to earth.
Despite Manette’s endeavors to use his influence as a former prisoner of the Bastille to free his son-in-law, it is Manette’s own letter, coming back to haunt him, that sentences Darnay to the guillotine. Manette would never consciously desire Darnay’s downfall. However, the fact that he is listed among Darnay’s accusers highlights the underlying rivalry for Lucie’s love that exists between Manette and Darnay.