#3: Do you show the right product image for the chosen product variant?
If added to cart products have different product variants, let's say different colors, product image thumbnail should always show the exact product variant that was chosen by a shopper.
Imagine that on product page you choose the black color of a product, add it to cart and then you see that product image thumbnail shows another color.

It can create a lot of friction in purchasing process, since it will not be absolutely clear which color you will get. That problem can happen if a product variant doesn't have a relevant image linked with it.

Good example from Shapermint:
You can see that each product variant has own image for the chosen color
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