I have known a couple of pit bulls who have suffered what one Vet called "happy tail syndrome." Both had long, narrow "whip" tails with only a thin coating of hair. Both dogs wagged their tails very fast and hard when they got excited.
One visited his human's workplace, where there was a very small office with just enough room for two desks and little more. He got overly excited and wagged his tail against the corner of a desk, splitting the tip of the tail open and flinging blood all over. The place ended up looking a murder scene! The injury required a Vet visit and a few sutures; the tail seemed to heal OK. It happened again a year or so later, so now the dog only gets to visit his friends outdoors, at least until he calms down a bit.
The other one was playing with a neighbor's dog where the two were separated by a chain link fence. That dog ended losing about 2 inches / 5 cm off the tip of her tail due to infection.
The odd thing here is that pit bulls are not one of the breeds that are traditionally docked.
