I've actually known two people who had very smallish versions of the degenerate twin thing. Both had them on their heads. One was my girlfriend of over 2 years, who had surgery for a tooth related sinus problem, that actually turned out to be one of these things that was lodged in part of her skull, and down into the sinus cavity, hence causing trouble when her adult teeth started to come in when she was 10. She has a small patch of plastic replacing a bit of her skull.

The other was my friend Seth, who had what was at first thought to be a mole going cancerous, and turned out to be a smallish one of these.

Weird in any event.