On a RJ, I had a FA fail to notice that my exit row neighbor was much too senile and too frail to even board by himself, locate his seat, or fasten his own seatbelt. (All of this was done by a traveling companion whom I assumed to be his daughter.) When I assessed the situation during takeoff, I quickly realized that I could easily just shove him out of the way in an emergency. I had a long conversation with the airline that evening, starting with the observation that his lack of emergency exit qualifying criteria should have been immediately obvious to the GA as he boarded as well as the FA if she had asked for a response during our abbreviated briefing on emergency duties.