https://www.medscape.com/viewarticle/902143

https://www.telegraph.co.uk/news/2018/05/08/american-boy-wakes-declared-brain-dead/
Twenty-one-year-old Zack Dunlap from Oklahoma appeared on NBC’s Today Show in 2008 to tell an incredible story of hearing a physician telling his parents that a PET scan confirmed that he was brain dead after a catastrophic brain injury. While he was being prepared for organ donation, however, he moved his arm purposely in response to stimuli. Dunlap recovered, went to a rehabilitation hospital, and ultimately went home 48 days later, very much alive.[1]
A teenager in Alabama who was declared brain dead regained consciousness a day before his life support was due to be withdrawn.

Trenton McKinley, 13, suffered severe brain trauma in a car accident from which doctors believed he would not recover.

His parents were told his organs were a match for five children who needed transplants, and they had already signed the paperwork agreeing to donate them when their son showed signs of life.