So, who's heard of teratoma's? No? Well, while you go Wiki, I'll cut through the crap and bring you the straight facts.
Essentially, a teratoma is a very special kind of tumour. However, unlike other, more cancerous tumours you may know of, this has a far more interesting cause. Essentially, these are the things that result when an embryo starts growing within its twin but then dies, causing the remaining twin to later have a tumor with hair, teeth, nails, bones, and sometimes even hands or eyeballs. And they can keep growing so that people don't really notice them until adulthood.
Just imagine having a third eye somewhere on your body. Or worse, being the trainee surgeon who opens the tumour to find it staring back at him.