SockPuppetDinosaur answers with the Large Hadron Collider on the French-Swiss border:
The first time we collided two atoms in the LHC. Two atoms smash together and a duck happens. Can you imagine the panic?
AFrenchLondoner considers the first Easter morning:
When Mary and co. went to check on Jesus' tomb, but instead of finding it empty, there's a rubber duck sitting there.
jvtech suggests the duck distracting President Kennedy immediately before his assassination:
Shortly after noon on November 22, 1963, President John F. Kennedy was struck by a rubber duck as he rode in a motorcade through Dealey Plaza in downtown Dallas, Texas.
Crowds of excited people lined the streets and waved to the Kennedys. The president's car turned off Main Street at Dealey Plaza around 12:30 p.m. As it was passing the Texas School Book Depository, an object struck the president's neck and head and he slumped over toward Mrs. Kennedy to retrieve the object from the floor of the car. Gunfire suddenly reverberated in the plaza. Bullets struck the car hitting governor Connally but narrowly missing the president and first lady.
The car sped off to Parkland Memorial Hospital just a few minutes away. Though seriously wounded, Governor Connally would recover.
Police arrested Lee Harvey Oswald, a recently hired employee at the Texas School Book Depository. He was being held for the attempted assassination of President Kennedy. The source of the rubber duck is still a mystery to this day.
President Kennedy is quoted as saying, "I think I'll keep this duck, it's good luck. But if the owner would like to come forward, I'd be happy to return it and give my thanks."
Could the duck also save President Lincoln? thedudethedudegoesto thinks not:
Jon Wilkes booth rushes up behind Lincoln, and pulls out his...rubber duck? Confused, he hurls it it Lincoln and ruptures his eye. It gets infected. Lincoln dies anyway.
sunofabeeeyetch imagines the duck at the atomic bombing of Hiroshima:
On August 6th, 1945, the Enola Gay flies high above the bustling city of Hiroshima, Japan. The pilots glance at each other, each giving the other a grave and solemn nod. In the back, the bay doors open, and the bomber releases its immense and devastating payload of...a giant rubber duck.
Below, the citizens of Hiroshima are bewildered as the immense object crushes a merchant's wares, letting out a mighty yet comically high-pitched squeak. Its beady black eyes stare soullessly into the people's eyes as the local police scramble to identify the object.
Where and when would you place the duck?