What is one way in which irony surprises the reader?

One way irony surprises the reader is by presenting an outcome directly opposite to what was expected. For instance, in the O. Henry short story"Gift of the Magi," Della sells her hair to buy a chain for her husband's watch, while Jim sells his watch to buy combs for Della's hair. The readers may expect the gifts to be perfectly matched, but the author surprises them with an unexpected chain of events.