A friend of a friend worked the tills at a racetrack. When they put money in the till, it somehow registers on the till and then you had to clear that before you read tickets to give money out to the bettors. The girl at the till forgot to clear the screen and then handed out $1000 more to a bettor than he was entitled to. She was on the hook for that money.
For me, if I discover a mistake in the store or the parking lot before I head home, I go right back in and fix it - either way (my favour or theirs). If I'm not far away when I discover the problem, I'll head back if it is more than $1 or $2.
If I get home, get unpacked and then realize the issue, if the difference is less than $10, I don't worry about it. It would cost me more in gas to go back and get it fixed. Because they are scanning errors (I don't do cash back), the till would still balance and the cashier wouldn't be out money. If it was a situation where the cashier would be out the money, I'd make an effort to go back. If it was a small Mom and Pop store, I'd mention it the next time I went in and fix it then. But a major chain doing thousands of dollars a day in business? I'm not going to worry about it, whether I owe them <$10 or they owe me <$10.
But if it was major, I'd go back to the store to sort it out. It might not be that night, but I'd do it. I have fibro so sometimes, by the time I get home, it is all I can do to make something for dinner. I'd be out of spoons if I had to go back out most nights.