Well, we know from experience that the Erins can get a little wacky tacky with their cat descriptions. But can a lot of these abnormalities be explained solely by incestuous genetic mutations? Let's take a look.
Clans, as we know, are typically wary of outsiders. Unless an outsider joins every once in a while, the gene pool is limited and many cats may turn out to be related to one another. This means that mutations are very viable under these conditions. Additionally, spontaneous mutations have been reported in many a feral cat colony.
Let's discuss some of the cats with abnormal traits:
The calico and tortoiseshell toms in the series can easily be explained away with Klinefelter's, but there's also a possibility for these cats to be chimeras which would lead to an ability to reproduce (IE: Redtail's case).
Spiderleg was born with the chocolate smoke trait, which barely exists outside of certain pure breeds, but could also spontaneously occur in a feral Clan where the gene pool is limited.
Heathertail has Ojos Azules traits, being a blue-eyed cat with solid brown tabby fur. This was originally a spontaneous genetic mutation found in a feral tortoiseshell cat named Cornflower, and was bred into the Ojos Azules breed.
Firestar would obviously be a cinnamon ticked tabby, again something that isn't an everyday occurrence outside of say the Somali or Abyssinian breeds. He's described as being solid, but the closest thing to solid would be a cinnamon ticked. Green eyes make sense in this way as well.
Anyone have any other genetic abnormalities that could be explained away by comparison to spontaneous mutations in the beginnings of purebred cats?