Or a wrong coolant/water mix and that could crack the block also. I'm sure a 3.1 has been cracked somewhere for all of these reason, but a thrown rod will not normally cause a crack in the block, most of the time it will cause a hole. Cause for a rod to let go, it must be under extreme over-revving or a seizing rod bearing, and as you can see for an overrevving engine not much will stop it from turning at that speed. But if an idling engine to get a seized bearing it will most likely just stall or suddenly stop. But if it is running a little faster, it will probably break and put it through the block. But in different cases will different engines let go.
But to tell this from experience, I've seen engines with holes in the block the size of a golf ball and still run for 10-15 20 minutes, but once they are stopped forget about restarting them! It will never happen!