You know less than you think you do. That's the only real premise of this book.
A man builds a device that talks his wife's cancer back into remembering it belongs to her body, and it buys her three years the doctors never promised. That's the opening story, and every one after it works the same way: something you'd stopped questioning, such as gravity, memory, a poem, your own reflexes, and then it turns out to have been lying to you the whole time, quietly, for years.
Fifteen stories move between a haunted manuscript, a Roman senate that never happened and shouldn't have, and a memory lab where a scientist can't tell if he's grieving or malfunctioning. Some are close to hard science fiction. Some are imagining the alternative history. None of them explains themselves before they're ready to.
If Exhalation by Ted Chiang left you a little unsteady on your feet, or you've missed short fiction that trusts you to keep up on your own - start here.