But for the positive effects of modern medication, James Danforth Quayle III, a man who misspelled the word "potato," would have become Acting President of the United States on May 5, 1991.

While the more notable situations of presidential disability have their own individual pages here, many of the 44 people who to date have served as President of the United States have had episodes where implementation of a process temporarily transferring their executive power to a Vice President (or transferring it under the 25th Amendment, in those cases since 1967) either might have been in the best interests of the nation, or at least was considered but ultimately rejected.  Some of these are noted below:

Chester Arthur

Having succeeded to the Presidency upon the death of James Garfield on September 19, 1881, within a year President Chester Arthur was suffering from problems of his own, specifically kidney ailments and various other abdominal pain.  By late 1883 he was also exhibiting symptoms of cardiac distress.  In 1884 he would only put in a token effort to seek a presidential term in his own right, aware that he likely wouldn't live much longer (he would die on November 18, 1886, twenty months into the 1885-89 term).

Ronald Reagan

Having invoked the 25th Amendment on July 13, 1985, President Ronald Reagan would undergo CAT scans on a regular basis throughout the remainder of his second term.  In July 1987, the President underwent a colonoscopy in which additional polyps were discovered and removed.  While Reagan was hesitant to invoke Section 3 in 1985 and would not do so in the July 1987 instance, in 2002 and 2007, President George W. Bush would do so immediately prior to undergoing a colonoscopy procedure which would prove less invasive.  Days after the colonoscopy, Reagan would have a small cancerous lesion removed from his nose, but the surgery would be performed under local anesthesia and thus, an invocation of the 25th Amendment wasn't considered.  In November 1987, the President would undergo prostate surgery under general anesthesia - during which time the 25th Amendment almost certainly should have been invoked, but wasn't; and just a week before the conclusion of his second term, the President would undergo another operation on his right hand under local anesthesia.

George H.W. Bush

On May 4, 1991, President George Herbert Walker Bush began experiencing shortness of breath, tightness in his chest and fatigue.  Diagnosed with an atrial fibrillation and hyperthyroidism, the President was taken to Bethesda Naval Hospital, where he was prescribed procainamide and coumadin.  The drugs were effective, restoring Bush's heart rhythm to normal within 36 hours, but had that failed his attending physicians intended to administer a cardioversion procedure to returning Bush's rhythm to normal - one which would have necessitated administration of general anesthesia.  Had that been necessary, President Bush intended to invoke the 25th Amendment and temporarily install Vice President (James Danforth) Dan Quayle as Acting President.  

Bill Clinton

While vacationing on the morning of March 15, 1997, President Bill (William Jefferson) Clinton tripped, injuring a tendon which connected his right kneecap to his thigh.  Clinton would be taken to Bethesda Naval Hospital where he would undergo a 90 minute operation repairing the tear.  The operation requiring only local anesthesia, it would prove unnecessary to invoke the 25th Amendment and install Vice President Al (Albert Arnold) Gore as Acting President.