You're really flying; phone poles flash by in a blur as you nail the pedal to the floorboard, and then it happens-oil pressure drops to zero and the thunder turns to death rattle as the oil film between the spinning crank and main bearings vanishes. Bereft of lubrication, it only takes an instant for heat to form a sticky bond between the crankshaft and the bearings before they start spinning in the block like some berserk lathe from hell. Congratulations, you're a pedestrian.
Spun main bearings are a disaster, but if you kill the ignition before things get critical and the parts hastily leave the crankcase, you can usually save the block. Here's a primer on how a quality machine shop repairs the damage.