What's a goathead?

I got some tubes pre-filled with slime at the beginning of the year. No flats yet, and I know I've ridden through glass and such. I think I broke a piece of a bottle on a ride the other day. (I'm very glad I wear sunglasses.)

I might notice a slight weight difference, but I suspect it's in my head. I'm guessing that if I do get a flat, it will be messy, especially if I have to carry the tube back from anywhere. Ew.