My dog, Kodi had a slab fracture on an upper pre-molar on each side - one in Sept 2005 and the other Sept 2007. Both teeth had to be removed. I don't know exactly what caused the teeth to fracture, but he no longer gets hard things to chew on.
Also, years ago, my vet said my 10 yr old dog had two cracked crowns. He said they almost always abscess and he was right - at least about the one. One morning I got up and the whole side of her face was swollen. She had to have it removed. I think the cause was bully sticks, but I can't swear to it.