Common sense dad would have said, “Cool, free knife!”
Fifty years ago common sense dad would say, “Great, now i have to return this knife.”
On numerous occasions over the years I've been under charged at Wal-Mart because of a case of Diet Coke under the cart and I always tell the checker it is there. I cut out the U.P.C. and the next trip to W-M, I take it with me, go to the service desk and pay for it.
The part I don't like about it is if they make a big issue of my "honesty", when anything less than that is stealing.
To be sure, if I am over charged, I let them know about that too but that rarely happens.