The difference between what is expected to happen and what actually happens.
"REALITY"
"Free! Body and soul free!"
IRONIC TWIST
Oh my goodness. That was close. I barely escaped with my life!
Dramatic Irony
The reader is more aware of what is happening than a character.
Honey! I'm home!
"Eeeeek!"
She was so thrilled that you were alive, it killed her.
While Mrs. Mallard is secretly reveling in the thought of her husband's death in the train accident, he miraculously walks away from it.
Everyone believes Mrs. Mallard died from the "happy" shock that her husband was alive. In truth, her shock was that of massive disappointment and sadness.