I cannot abide the horse that bolts. I don’t mean the silly little spook-n-scoots that any horse with eyes and ears might do. I mean the criminal-minded, take charge and give you the California howdy type of bolt. I don’t care how cheap it is, or how fancy, or how well bred. If I get a whiff of any of that, either from digging around in the horse’s history, or on a trial ride, I’m done. That being said, I also avoid horses listed as “quirky” - because quirky usually means dangerous in the wrong situation.
Physically, any serious arthritic changes in a horse that is less than 9, feet that require complicated shoeing, hind suspensory injuries, or neurological presentations/history. I can forgive a lot of bad x-rays if the horse presents as clinically sound and has no positive flexions.