Good topic!
Many many many...
I once dropped a distributor mounting bolt down in to the block, on to the jackshaft, it then fell in to the sump. I had spent three days putting the sump back on as the threads of the sump mounting bolts near the subframe are damaged. I had to walk away and didn't return to it for a week.
Two weeks ago I knocked the fuel feed pipe of one of the carbs, and petrol poured everywhere, had this occurred when driving I have little doubt it would have caught fire and burned the car to a cinder.
Set the timing and camshaft to crankshaft 180 degrees out, somehow. I had checked and checked it. The engine did not like it.
And I once dropped a large G clamp on to the front eyebrow panel, the dent is still there.
Bad dolly owner.
