What would you do if a large retail store made a significant error in your favor?

I bought a rather expensive pair of boots on Friday. I was writing a check because my deposit was due to hit my account that day but hadn’t yet.

This store processes checks electronically, which typically takes up to 24 hours for it to show pending on your account. There was an issue with their computer during the transaction. At first, the computer wouldn’t recognize the boot in their stock. So she rang up a boot that was priced identically. Then she tendered payment, and something happened and she felt she needed to double check that it went through. The register wasn’t producing a receipt, and she got frustrated and said “do you need the receipt?” To which I said no because I didn’t intend to return the boots. She gave me the check back, as stores do when they process them as e-checks.

It’s Tuesday, and the socks I bought at the same time with my debit card as a separate transaction are there and fully processed… But there is no trace of the boot transaction. I think I just got $180 boots for free.

What would you do in this situation? Would you remedy it? If you would, what are your reasons? And if you would just count free boots as a total score, why?