Krogstad's crime is ironic because it is, essentially, the same crime that Nora Helmer commits. Nora has committed a forgery, signing a contract with her father's name (as a woman at the time was not allowed to borrow money without the express permission of her husband or father), and Krogstad becomes aware of her forgery because she dated the document three days after her father had died. Krogstad tells Nora,

I can assure you that my one false step, which lost me all my reputation, was nothing more or nothing worse than what you have done.

It is highly ironic, then, that he is judged so harshly by others, others who are absolutely blind to the fact that their beloved Nora has done something so similar. Dr. Rank, who thinks so highly of Nora—who, in fact, loves her—feels that Krogstad possesses a "diseased moral character," and yet his beloved Nora has actually committed a similar crime, though one Dr. Rank is not aware of.

Torvald Helmer, Nora's husband, calls Krogstad a terrible "hypocrite" for trying to get out of being punished for his crime of forgery, when Nora has been attempting to do just that herself. For years Nora has kept a secret from Torvald: he knows nothing about her forgery or the illegal loan she took out from Krogstad, making her just as guilty of wrongdoing as Krogstad has been. Krogstad, ironically, is judged incredibly harshly, while Nora is never even suspected of wrongdoing, because as a woman, she is infantilized and condescended to by her husband. It is only after Torvald learns the truth that he realizes Nora is even capable of deception greater than sneaking a few macaroons.