There’s a really interesting article on Fran Jurga’s Hoofblog today about nerving.
In the case she discusses, a British race trainer was suspended after racing a horse that he’d had nerved.
Over the years I’ve heard stories of people buying horses that turned out to have been nerved. It’s very difficult to tell if a horse has had the procedure done without testing to see if there is any feeling in the foot. The horse will likely move just fine (the nerving will have eliminated the pain that prompted the procedure) but riding a horse that has no feeling in its foot (let alone racing one) is foolhardy.