I have two of them for you.

Florida, ~78

I was a teenager, riding a water buffalo. Time to change the fluids.

I drain the transmission and radiator, refill the transmission with motor oil, then start filling the radiator.

As soon as I start pouring antifreeze mix into it, it starts coming out the bottom. I'd forgotten the plug. I look underneath, and had forgotten the transmission drain plug too, but that oil takes a bit of a path to get there, so it was just then pouring out.

Missouri, ~84

My father bought an old Chevy wagon. A '66, I think, for $250. It ran, drove fairly well and had new tires.

After he had that car for a few months, we were motoring along (I think he was driving) and smoke started pouring out from under the hood. Shut down the motor and popped the hood, and motor oil was drooling off of absolutely everything. The underside of the hood was dripping with it, and it was evaporating off of the exhaust. No fire, thank goodness.

The car had burst a radiator hose. I noticed that after much looking around; there was a small split in one of the big hoses, and the radiator had been filled with motor oil.

I bet some of the old-timers can see where this goes. There is only one reason to fill the radiator with motor oil, and that is because it has a cracked block.