Racehorses:

Randolph M. Nesse:

"Racehorses are prone to breaking the cannon bone in their legs. Why didn't natural selection make it thicker? It did; wild horses are unlikely to break their legs. However, breeding only the fastest horses made their leg bones longer and longer, thinner and thinner, and lighter and lighter. Successive generations of racehorses have become faster and faster but also more and more vulnerable to breaking a leg, something that now happens about once every thousand times a racehorse starts a race."