I was surprised to discover that the bit about Dorothy falling asleep in the poppy field in _The Wizard of Oz_ is based on a real risk--opium fumes hang close to the ground in poppy fields and children shouldn't go into them. So much else in the book is unrealistic, and I wasn't in the habit of thinking of flowers doing biochemical warfare.
In King's _The Green Mile_, there's a bit about a child's fingertip regenerating after being cut off. All the characters accept it as a miracle, and if I hadn't read _The Body Electric_ (a book about electrical fields in living organisms), I would have taken the regeneration as a fantastic element. Before some lowish age (8 years old, I think), children have the ability to regenerate fingertips.
What else in fiction is implausible but true?