We can anticipate that the amendment will sometimes work
just fine. If the president falls into a coma, the vice president and “the
principal officers of the executive departments” (presumably the cabinet) will
certify this fact to the speaker of the House and the president pro tempore of
the Senate (now always the majority leader), and the vice president will take
on the presidential duties.
What, however, if the president is conscious and functioning
but seriously mentally impaired, an incident of that impairment being an
inability to recognize it. The president might then fight a 25th
Amendment certification.