Is there a word for the situation where you are expecting something and then something totally different happens

Plot twist

A plot twist is a radical change in the expected direction or outcome of the plot of a novel, film, television series, comic, video game, or other work of narrative. It is a common practice in narration used to keep the interest of an audience, usually surprising them with a revelation.

https://en.wikipedia.org/wiki/Plot_twist

In certain contexts this can be shortened to twist.

Shyamalan said in 2006, "I was hesitant . . . because the book has kind of a twist ending."

Consider, coup de théâtre (also spelled coup de theatre)

: a sudden sensational turn in a play; also : a sudden dramatic turn of events or effect

Origin: French coup de théâtre; literally, stroke of theater. First Known Use: 1747 M-W

: French term meaning "stroke of the theatre." A sudden, startling turn of the plot used to effect great surprise. McGraw-Hill Encyclopedia of World Drama

Consider turnaround.

Definition: a complete reversal of a situation or set of circumstances; a complete change from one way of thinking to an opposite way of thinking, etc.

Example: The writter was sentenced to be hanged for loving the king's daughter. Just before his execution, he composed the poems and so impressed the king that there was a lucky turnaround for the poet and the king gave his daugther to him in marriage.

ironic: "happening in the opposite way to what is expected"

e.g. John expected to dislike Jane's latest boy toy, Jack. Ironically, the two men became close friends.