Timely discussion for me! I posted (72 Demon Follies) an issue I had at the track with my avatar car this past Sunday. 408, 10.5 CR, Eddy RPM heads, MSD 6AL. Car runs 7.0's/11-teens.
Mph seemed slower than I thought it should be after 1st run in the DA air of 73. Pulled the Champion RC12YC plugs and found #3  like this:
No center electrode! All others were ok.
I knew I might be a tad lean but not lean enough to do that! And there was no nibbling or pecking on the ground strap so it's not like it was detonation or anything. And #3 has the center electrode smooth and even with the porcelain. #5 electrode is sunken into the porcelain about 1/8". Pics aren't great but reading the ground strap on all the plugs showed the timing to be just about spot on.
Changed out the bad plugs and made additional passes. Set a new Personal Best of 7.01 in the 1/8th so the motor apparently was fine.
Fast forward a couple days. Through some connections, I was able to get the pics back to Champion and learned the design is an older design and the sealing mechanisms have changed quite a bit since that plug was designed. The "center electrode pushback" like I experienced is a possible failure mode of the design caused by high cyl pressures. I doubt mine is excessively high but apparently was high enough to do this. And I'm sure many folks have run them forever without issue. I'm just relaying what occurred. The recommendation was to use a modern design like RC12WYPB4 (#9201) in place of the RC12YC.
So I'm putting new plugs in it this week. And given the big plug gap the MSD box was apparently pushing through, I'll check my plug wires carefully to make sure no breakdown occurred. Live and learn!!