Most likely a genetic defect. The goldfish that are sold in pet stores are culls from breeders of show quality fish, and will usually have some small "errors", such as: Bended fins, single pelvic, and anal fins, different sized caudal lobes, thinner bodies than the standards, differently colored eyes.... etc etc etc

REALLY?! I've never seen any such things here! First of all, why do they even try to sell those? Most people would have troubles taking care of disfigured fish with special needs :/ All of the fish around here have been really nice fish, although obviously not show quality.
At most, when I had guppies as a child, they were breeding and breeding and breeding, so I couldn't tell which one was the mother, father or brother to which xD In the end I had a few that had bended spines, just a bit bended, but still visible. They had no problems anyway and lead very happy lives

Yes, it's awful. They buy culls for super cheap. I guess some people don't know very much about fish or don't take the time to look 'em over very carefully, and chain stores take advantage of that just to make a profit. I think severely disfigured fish, and I mean VERY disfigured, should not be sold at all. Petsmart takes pretty good care of their fish around here, but the ones they buy are not healthy. That's why it's so hard for me to get a good fish around here.  And funny about the guppies! When I had some they gave birth ALL THE TIME. However, the babies all got eaten up...canablistic fish.  Walmart takes horrible care of their fish, although they don't buy bad culls. If you catch 'em right as they get a new shipment of fish, before the fish have time to get sick, you can sometimes save them just in time. And Ireiden, good guess.  you'll get used to the fish lingo around here.