The Father of Our Country Falls Ill
The "Father of Our Country," George Washington, contracts a severe case of influenza. While some accounts of the day describe his condition as "near death," by any account the first President of the United States isn't up to the job for a period of several weeks.
While Washington is laid up, Vice President John Adams would prove even more powerless than the Commander-in-Chief. Less than a decade removed from the American Revolution and a scant thirteen months into the first presidential term in the nation's history, a temporary transfer of executive authority would be in the best interests of the nation.
But because there's no clear mechanism in place, and in no small part because public perception might see it as a de facto coup d'etat, nothing happens.
Ultimately, Washington recovers from his illness. But just as his retirement after two terms of service would establish a precedent, so would his illness in 1790 - and while the two-term precedent would end in 1941, this one would last 197 years: the precedent wherein short of the death of a President, a Vice President of the United States is powerless to wield executive constitutional authority, even on a temporary basis.