Close nailing means you got the nail too near the sensitive part of the hoof. If a horse has thin hoof walls, it is easy to do. It CAN cause an abscess but usually, if you find the horse lame after 3 days, the solution is to pull the shoe, use hoof testers and make sure there is no abscess and reset the shoe.
I had a horse close nailed a couple of times and pulled the shoe and then reset it myself and all was well.
An aside: I used to shoe my own horses and then started having a farrier after a knee injury. He was a good farrier and because he knew I could take care of putting a lost shoe on and so forth he gave me quite a break on the price of shoes. I rode the horses so much the shoes were worn out by 6 weeks.. and always needed new shoes ($4 extra). Never did any resets other than a lost shoe or close nail. He always shook his head when he saw the shoes and said I was one of the few clients he had that really used their horse.