Heatrocks can short circuit just like any other electronic device. Either they go out completely or can heat up. Most reptiles can't really feel heat fluctuations as well as we can. So if the rock heats up they can get their insides cooked. When I first kept lizards I used heat rocks but have since then used under the tank heaters. I'd rather not take the chance of them being cooked. And besides, heat pads aren't that expensive.
