There are a few who are unsuitable to be pilots. One that I had to chop had a severe problem with narcolepsy - in other words he used to fall asleep during the lesson! Another guy suffered from inability to recall what he had been taught four days previously and used to do things in a haphazard way based on the fact that he knew he should be doing something at a certain point - eg. retract flaps at 300 feet on climb out - he would reduce the throttle etc. (Environmental shift?) Another could never overcome his nausea and threw up every time we descended to land. Clearly all unsuitable to continue training.
Incidentally, my first instructor wrote in my student records that I would never learn to fly! Here I am 20 years and 7500 hours later, having trained over 150 instructors...but there we are - some 24 year old boys have remarkably little ability to judge the potential of older women and display their immaturity and lack of teaching ability by yelling, shouting, abusing and showing off to their students. It is all about empathy and patience.