We had one years ago who did a fracture behind. Was operated on and was ok till a week later. Broke the other leg trying to kick the cast off! Was a damn good mare too - was on course for the Stayers hurdle.
He was jumping a collapsable fence downhill, hit the top pole, which rolled down the hill, landed on it and snapped both his front legs.
He stayed on box rest for months as it did start off looking quite promising, but then it went downhill, and after months and months of standing stock still with casts up to his armpits on both legs, they made the decision to let him go, his legs just weren't healing.
Not now but he survived both fractures! There are so many options available a broken leg is not necessarily the death sentence it once was. Any open fractures or badly displaced ones carry a poor prognosis but even these can be saved just look at Barbaro www.vet.upenn.edu/newsandevents/news/Barbaro_Updates.htm