This came to me in a vision in my sleep a couple nights ago.
What is going on here? Well, catchlights orientation. Look closely at the 'wrong' one, notice how the white shines in his eyes are mirrored, while on the 'right' one they are not? This is a common mistake people make, especially with forward facing but can affect other angles too. If you mirror the pupil and iris, you need to remember to flip the catchlights so it looks proper. It's a small thing but it makes a huge difference in how things look.

Now, I'm not perfect, I consistently made this mistake for over a year and a half, but I learned, I try to make sure I don't mess up anymore. I might still have a few vectors that are incorrect, and I'll admit mistakes happen. But I hope I can teach at least one person to pay attention to this now. I can't turn off my eyes for noticing it in other's artwork now, for better or worse.

I just chose a super basic sitting 'looking at you' pose to use as an example for fun.

No tutorial version.