I got five in a row without looking for glitches or background objects, then stopped.
Look really carefully at a small area of skin. See if wrinkles, pores, hairs, and minor skin imperfections are present. See if they make sense in the context of the rest of the face.
Not the face itself, bur what's around it/background/other objects etc.
Check for weird-looking "something's not right" objects/backgrounds and you'll get most of it fine.