Long sleeves should have the same length.
Careful while flipping the image! It can ruin it completely.
If the brand is important, brand labels are important, too, and should not be wrinkled.
The backside of the collar should not be curved upwards unless it’s the design of the product. The brand label should not be stretched.
Products should look symmetrical unless it’s the design of the product.
Mannequin parts and seams should not be visible under the material.
The "ghost mannequin"effect is not possible if you can see plastic through the garment.
Waistlines of pants and underpants should not be curved. A straight line is the best approach. Crotch area should not be wrinkled or weirdly shaped – that is, if you can afford it, as it's time-consuming. Maintaining a straight waistline is necessary all the time though.
When several products are combined in one image, their proportions should not be messed up.
No props or pins should be visible.
Flat lays should not be wrinkled unless it's on purpose.
There's no one single approach to how thoroughly mannequins should be retouched. In some online stores, the left image would be totally fine, while in the others only the right image would be acceptable.