What is it called when there is a difference between what one expects to happen and what actually happens?

The name of this is Situational Irony

Explanation:

Situational irony is probably the most common kind of irony that a reader will find in literature. The authors use situational irony to surprise, intrigue and engage the public. When the reader expects something to happen, but something else occurs, the reader can then reflect on the decisions, motivations, and relationships of the characters and events that happened in the story. Most people are familiar with a "plot touch" in a story; This is usually when the author employs situational irony. For example, in Great Expectations by Charles Dickens, situational irony occurs when it is revealed that Miss Havisham is not Pip's benefactor, despite having led him and the reader to believe that it is. Instead, Pip discovers that his benefactor is his condemned, who has no station or property and is a fugitive in England. This twist of plot surprises not only Pip, but also the reader, and leads to a new adventure for Pip at the end of the novel. Situational irony differs from dramatic irony in that the audience and the characters are not aware of the truth that is revealed in the plot.