P.S. I should add that not only can I not get the nail out of the ewall, I also can't drive it in further. Something is blocking it from going in, and something is grabbing it and not letting it come out. (Do I have a house gremlin?)

Take a block of wood and place so when you hook onto the nail, you are prying against the block at the nail head. This way you also won't damage the wall, and you can about hang on the hammer if you need to. Gives you much more leverage this way.

You probably got it into knot. Amazing how much they hold a nail, if you can get it into them in the first place.

Try to make sure to get the nail seated firmly, it gets real fun if you round the nail head off, and then have to use visegrips and a puller to get it out.

You can also try a 'wonder bar' and some blocks of wood.

Make sure the wood is on the stud, or you can easily punch through the drywall prying.

If it is a finish nail with a small head a pair of 10 inch vice-grip pliers clamped tightly on the nail gust below the small head should grab enough to get the nail out.
Pry on the plier jaws with a wonder bar.

Since you're painting anyway, why not just take a razor knife and cut a square piece out of the plaster carefully. It is easy to restore with some glue a piece of flat molding and a piece of string. Just put glue on the ends of the molding after tying the string to it. Pull it back against the wallboard and let the glue set up by tying it to another piece and twisting it until the outer piece is flat against the wallboard. When dry insert the piece of wallboard in the hole and attach with wallboard screws. Patch the crack and sand and your ready to paint. That way you will see what is going on and get the nail out easily. Total extra time of about an hour except for getting any materials that you don't have on hand

Generally, if you hit something really hard in a wall, it was put there to stop you. eg a plate over a water or electrical line.

BTW- anything you nail in CAN be pulled out. You just have to pull hard enough and it might require the blocking to keep from knocking a hole in the wall

Hi, Grab the head of the nail with some pliers, bend back and forth till the nail breaks patch the small hole and paint, You weren't trying to save the nail were you?
Good Luck Woodbutcher

What woodbutcher says or if you have a dremel tool, put on a metal cutting disc and cut the nail off a tad into the wall, patch,paint.