In the case of roses, I think it's a common occurrence, but most gardeners don't bother to register them or try to market them. It's a long and complicated process, and you'll find that most of the attributed discoverers of sports are hybridizers and rose sales companies. The average gardener doesn't have the knowledge, time, or money to invest in registering a sport.
Cliff Orent discovered a wonderful sport of Rosie O'Donnell, which he called Trump Card. It's a stable sport. I've been growing it for a few years and it has never reverted to Rosie. He looked into marketing it and Weeks offered to test it and consider it for marketing. After testing it for a while, Weeks decided to put its money into Wing Ding that year instead. I personally think that was an awful decision. I don't know how typical I am as a rose grower, but I buy a lot of Weeks roses. In spite of my shortage of space, I have three Trump Cards in my garden and not even one Wing Ding. My Trump Cards came from Cliff, but I'm certain the rose would be a huge success if it were readily available in commerce.
