Can there be sudden failure of fan clutch? No indication of any trouble up to incident below.
Testing/driving a couple of miles in suburban traffic, stop sign/light every couple of blocks, after repair of throttle body "heater" gasket failure so checking temp gauge constantly. Temp never varied from normal. Stopped and let engine run a bit while listening to end of something on radio.
Bang! Cooling system puked through the radiator cap.
Shop replaced fan clutch and everything then OK.