maineguide:
If you left enough lubricant on the cases to dent them on firing, you would get wet fingers loading them! More likely a bit of gas has made it back past the case-mouth seal before the bullet exited the muzzle. This doesn't happen very often with normal brass elasticity and normal pressure loads, and is more commonly seen on the shoulder of bottle-neck cases. I've only ever seen it happen on brand new brass.

Is it a problem? Not unless it happens again IMHO...

Hopefully the cases will pop out on resizing, or at next firing.