The common words for this mistake are to puncture, to pinch, to prick, or to quick a horse. A horse that has been “quicked” is one that has had a nail driven into the sensitive area of the hoof. The result of this is the immediate, within a half hour of shoeing, lameness of your horse. I say this simply because there are horses that will react to pain instantly and others that show little signs until later. One mare I do has a club foot and cannot take a heel nail half of the time, the other half she’s fine. She also needs as many nails as can be applied because she tends to pull shoe easily. If you accidentally lodge a sliver underneath your fingernail you certainly are aware of it immediately. Yet there have been cases where a horse has gone lame five weeks after shoeing and somehow it was though to have been “quicked”. That is an impossibility! In fact, even two or three days after shoeing, if the horse goes lame, it is unlikely that the horse was quicked. It may well be another shoeing fault, e.g. improper loading of the hoof, but probably not nailing.
Once a nail has been driven into the hoof and the horse moves about freely without any signs of lameness, then that indicates safe nailing to the owner. If the hoof is thin walled or had a shoe off before, there’s a chance that the nailing will be a little close but will improve gradually during the next 3-5 days. The alternative is to leave the shoes off and stall rest your horse for the next 2-3 weeks, waiting for growth. Most owners wouldn’t be too enthused about that prospect! The nails are tooled so that once they have been directed into the hoof the proper way they will never change direction back into the foot.
A common cause for quicking a horse, outside of carelessness, is trying to shoe a hoof that is worn down to practically the bloodline. The advice to rest the horse for 2-3 weeks is usually not acceptable (as has been mentioned) for this or that reason, so on goes the shoe. If the horse is quicked the workmanship cannot be blamed. At least the farrier tried. Boots can be used for 4-6 weeks, plus Biotin or other supplements started to speed growth. The owner has the last say, but will often go with the advice of their farrier, whom really would like to see the horse up and going as much as anyone. These situations are easier to handle when the client and farrier have a history and mutual trust has been established.