Both Darcy and Elizabeth, at these separate balls, have given in to their negative perceptions about the other person, and have revealed their main character flaws that will keep them apart for so long. At the first ball, Darcy reveals his prideful character; he acts aloof for the entire ball, and his comment about Elizabeth is very stand-offish and snobby: "She is tolerable, but not handsome enought to tempt me; I am in no humour at present to give consequence to young ladies who are slighted by other men." Here Darcy reveals his tendency to make snap judgments, and for his pride to overwhelm any other senses. When he does finally come to his senses and move past his pride to see Elizabeth's true value, she has been tainted by Wickam. She comes to the next ball with prejudice against Darcy reigning full in her heart. It overwhelms any other possible sentiment. So it is now she that is the one who misses an opportunity because of a weakness in character.
The irony is significant because it is an equal situation in which each one of their flaws-pride and prejudice-are seen clearly, and to the detriment of them both.
The irony of Elizabeth's actions hits Darcy over the head. He is not used to women refusing him, he is accustomed to women falling all over him because of his social position and financial status. Elizabeth, a gentleman's daughter, but a pauper compared to Darcy, has the nerve to reject his attention toward her. This is unthinkable for the prideful Darcy at this point in the story.
But what is particularly interesting is the way that Austen writes these characters, giving Elizabeth Bennett a personality that allows her to reject a rich, rich man even though her family is desperate for a good solid marriage from her or Jane. Elizabeth 's integrity, honesty and devotion to the truth are the main reasons that Darcy falls head over heels in love with her. She does not grovel at his feet, even though, she, more than Caroline Binghley, needs Darcy's money and position.
It is very ironic for Darcy, then, that Elizabeth sticks to her principles, rejecting him for his arrogance, even though Darcy is very rich and could be a savior to her family's situation.
The other really funny irony that is very common in every day life is, just because Darcy changed his mind does not mean the opportunity still exists. How perfectly wonderful that Darcy rises above his original behaviour and IS now more than willing to give consequence to Eliza Bennett.
I have always believed though don't recall reading it, that Darcy spoke so thoughtlessly when he didn't want to dance with Lizzie, that most likely he didn't even recall the incident.