The flywheel cannot come off if the bolts fail, it is physically held in place between the crank spigot that it sits on and the input shaft of the gearbox (ask Rod try - he sheared the bolts behind a 2T and his legs are still in place because the flywheel never left the bell housing).
Feet get smashed when flywheels and or pressure plates explode (pressure plates to a lesser extent because the clutch cover contains a lot of the damage).