Can I get some advice on an odd render effect?

DDCreate

I rendered a dozen pictures over night and two of them decided to do this weird Terminator metal skin...thing. It's happened before too and it's kinda galling. Does anyone know what causes this and how I can prevent it? They have no skin effects like wet skin or sweat or anything. It's just three people in a meeting.

DDCreate

This one turned out fine. Just for reference.