Back in the early 80's.
While driving home in my 1954 Chevrolet panel truck,
In the corner of my eye, I saw the oil pressure gauge needle suddenly drop to ZERO.
I had about a 1,000 feet to go before I was at my driveway.
At about 500 feet I shut the engine off and coasted the rest of the way.
It turns out that
This Fitting snapped Off. (oil pressure gauge feed line fitting)
Leaving just the threads in the block.
I did what
tclederman did and used a easy-out to back out the threads.
It was easy to get out, no pun intended.
The only thing that was bad about the whole problem was the 500 foot
streaking path of oil on the street.
Hope that that snapped off plug didn't screw-up your paint job!