My brother dropped a washer down the intake manifold when he was swapping carb base gaskets. He thought the washer fell to the ground, not down into the engine. Long story short when he started the engine the washer embedded itself into the top of the piston, cracking it all to pieces.
The engine was totaled, he could not afford a used one, and sold the car for next to nothing.
Moral of the story is take as much time as it takes to correctly remove the bolt your searching for, before rotating the crankshaft or starting the engine as has been recommended.