I was watching a tutorial the other day on retouching with lightroom, and they showed on effect that was handy to make skin better (smoother, fewer wrinkles, etc.) and said "don't over use this, or you will end up with plastic people". They then tweaked the control to the max, and instant mannequin look.
But I can't remember which control it was. I've got vague memories of it being de-noise or clarify but I could be 100% wrong in my memory