Cutting a dog's toenails too short damages the quick, which has nerves and a blood supply. It is painful to the dog, and the quick bleeds when cut.
When trimming dog toenails, it is critical to pay attention to quick length. When nails grow overly long, the quick also grows long and is easy to accidentally cut. The quick is the pink center visible in light-colored nails. In dark-colored nails, a small horseshoe shape appears in the cut end when it is near the quick and cannot be cut any shorter. A cut quick may bleed profusely, so dog owners and groomers often keep styptic powder to stop the bleeding.