The attack on New York and Columbia in the sky.
Did that really happen or were they just dreams Booker had?
Of course that scene with granny Elizabeth talking to him in the mental asylum thing shows it again.
But if such an attack were to have happened for real would those stupid airships be a match for the Air Force?
And howcome for 84 years no one bumped into the city while flying in a plane or spotted it on radar? They're still using 1912 technology in the attack so to take over Columbia would be no contest..