Actually happened to me from the seller side about a month ago. Sold a ~$100 T206 in a SGC holder. It was in a stack of mainly SGC, but a few PSA. I grabbed the sold SGC holder, and wrapped it in shipping form, card-boarded it up and shipped it out.
Less than a week later the buyer emails me saying he loved the SGC, but why did I send him a ~$200 PSA card ???!!! I must have grabbed both (being PSA much thinner), and sent them both.
Very lucky I was dealing with an honest buyer, for I would have never known --- unitl the PSA card sold, I couldn't find it
He sent it back, all's well that end's well
I think, or like to think, most hobbyist would do the same (in either direction).