I once got to ride my instructor’s I-1 horse. And a fussy mare at that! Happily, didn’t “ruin” her! I did get changes I didn’t know I asked for as well as a bit of passage and an unscheduled halfpass. We both survived and I learned a lot about body control and demanding mares.
I don’t believe an inept ride will ruin a horse that has been well trained. Only real abuse will have a lasting effect. I also agree that some horses are more tolerant and resilient than others. And with some horses repeated inept rides will erode training pretty quickly. That is why we value schoolmasters who patiently wait for their new student rider to get it right!
