What scientific discovery would change the course of humanity overnight if it was discovered?

For me "overnight" is a short period of time. Something hardware such as a method of generating cheap, clean and abundant energy would take years to phase in to industry. The same goes for teleportation or room-temperature superconductivity.

An example of hardware that might come close to changing the course of humanity "overnight" is discovering a method to move backward in time. Even if there was only one time machine and it could go back in time before the time machine existed; just that alone would make humanity reevaluate the concept of time and cause/effect. People would debate what "history" really is if it can be rewritten. Nobody would ever again say that mistakes are in the past and there's nothing that can be done to fix it.