Other mistake: When Stephen Hawking's life is played backwards and then rewinds at the end of the film, the sequence of his sudden illness at the concert and subsequent operation at the hospital are the wrong way round. It should show him having the operation, then winding back to his collapse at the concert, but the two events are in the wrong reverse order.
Continuity mistake: Soon after Jane meets Stephen at the beginning of the film she writes her telephone number on a napkin. Later when he looks at the number it is written much more clearly.
Factual error: The view from the aircraft Jane is flown home from France in shows the modified double engine pod of the 747 famously at Dunsfold Airfield, Surrey. Also a late livery grey RAF VC-10 parked in the distance. Also the engines of the plane she is in are upgrades and not right for the year it is supposed to be happening. (01:23:20)
Chosen answer: Throughout his existence, prognoses of the life expectancy for Dr. Stephen Hawking have been repeatedly offered and then surpassed. He continues to amaze with not only his brilliance, but his resilience. In this case it sounds like a callback to the initial estimate, which was demonstrably inaccurate.
Michael Albert