And I'm not talking about big things like a TV or other electronics or something. If I walked out and realized that my receipt was $53.89 instead of $453.89... I would feel bad, and probably be paranoid that they would realize their mistake and blacklist me from the store or something if I did not pay for it.
How would that actually work in the US?
In the UK If I purchase a TV that is worth £500.00 but it scans at £50.00 if they do not notice the pricing error before I pay for the item and get the receipt then legally the item is mine and the store can do nothing because a contract has been formed and agreed at the £50.00 price point.
As for feeling bad about it, it would depend on the type of store, if it was a small one man band type operation then I would be honest about it because they need the cash. The larger chains have these sorts of expenses built into the cost of sales so when you buy an item a small fraction of that is to cover expected losses anyway so I would not care.