Stop in for a cup of coffee

Ok. Start this motor, it runs. One pull. Rev it , idle it. Drops a cylinder. Wtf??
Pulled plugs, one is like new,other has a little soot.
Bottom one seemed loose, but hard to tell as my socket binds in the well. So, stuck it in lathe and took .040 off and re-installed plugs. Has been running 20 minutes and hasnt skipped a beat.