I once purchased some plants at a home improvement store and was chatting with the clerk about what a nice day it was and not paying attention. DAYS later I noticed that there was a $99 birdbath as the first item on the receipt. Well I had no way of proving I didn't buy a birdbath, so I never complained, but I always wondered: (1) did the clerk plan it and pocket the $99? or (2) did she do a pricecheck for someone and not clear the register so it was an honest mistake?. I'll never know but I learned the valuable lesson you just spoke of. Always check your receipt!