President John F Kennedy's assassination in Dallas on 22nd November, 1963, shook the world - but the fallout from it would have been far greater had his successor, Lyndon B Johnson, been killed on the same day.
Just hours after LBJ had assumed Presidential office after being sworn in on Air Force Once in the immediate aftermath of JFK's assassination, the new President of the United States was almost shot too - and by a Secret Service agent, no less. With the Secret Service on high alert following JFK's assassination, one particular agent named Gerald Blaine was patrolling LBJ's estate in Washington that evening when, out of the corner of his eye, he spotted a figure walking around the corner. Blaine readied his machine gun and put his finger on the trigger, ready to fire - until he finally noticed it was a visibly-scared President Johnson, who quickly rushed back into the house.
If Johnson had also been assassinated on the same day, then the Speaker of the House is constitutionally the next in line for the American presidency - and in November 1963 that was 71-year-old Democrat John William McCormack. It's fair to say that, if McCormack had been elevated to the Oval Office, the USA's future - having just endured two presidential assassinations in a single day - would have been very, very different.