* Bent valves: The pistons can collide with the open valves, bending or breaking them. This is extremely serious and requires a major engine repair.
* Internal engine damage: Other internal components can be damaged as well.
* Engine seizure: The engine may seize completely, making it impossible to restart.
While in some extremely rare cases (usually interference-free engines), a car *might* briefly crank over with a broken timing belt, driving it is almost guaranteed to cause catastrophic engine damage. Do not attempt to drive a car with a suspected broken timing belt.