The nut slot for the G string was a bit wide, and I got a pinging sound every time I played that string open. The ping comes from the section of the string from the nut to the tuner peg.

I took some superglue and baking soda and QUICKLY mixed it up and dabbed a bit in the slot. Then I took a string and evened it out real fast in the slot. I got too much in there of course, so then I took a tiny piece of very thin sand paper, which I folded in half. I "filed" with the sandpaper a bit, and pushed down with the string while the glue was still not super super hard.

The result is no pinging, the nut slot is less wide, and it got bit less deep too. If I had a real nut file, that would be the way to go, but this still works. The guitar stays in tune and no pinging any more.