Note the words on the foreheads of their helmets :: they render OK on the man on (your) left with the faceshield, but with the bottoms of the letters shaded on the man on (your) right. The likeliest cause that I can see is that the man with the faceshield 's helmet forehead goes straight down, but the other man's helmet forehead flares out into a peak, plus faulty shadowing in the rendering software.

Looking at it closely it sort of looks as if the words are placed on a decal, so it might have something to do with transparency and ray bounces. Have you tried increasing the transparency rays in your render settings?

I realized that also, later. Thanks. I have cured the trouble.