What happens is the opposite of what was expected, or of what someone planned, in a way that reveals something about the characters or about the world of the work.
In The Magic Mountain
I have arrived at the very firm opinion that you are, secretly, one of ours and would end up realizing this, as so many others did who came here to amuse themselves, studied the atmosphere, turning up their noses, and one fine day found out that it would be convenient for them [...] to abandon their attitude of careless curiosity and spend an extended stay here
Hans came as a visitor, "to amuse himself" or to look on with "careless curiosity," and the doctor tells him that, in truth, he has always been one of them. The short outing turns into the opposite of what he planned: a long stay on the mountain.
Chapter IV: The Sense of Time, Hippe and the Thermometer, Dr. Behrens
What he had dreamed was on the point of fading away. And what he had thought, that same night, he no longer understood very well
Chapter VI: Naphta, the Snow and the Death of Joachim, the narrator
Subscription
The explanation of each passage, in every device and in every work, comes with a subscription.