I try to keep track also. Mervyn's was famous for having things marked at the rack but not on the register. If you didn't watch, the clerk didn't either. They were actually caught and severely fined for their actions.
And I have contacted a local fast food restaurant on-line when I was short changed by $5.00. It was not for the $5.00 but I was concerned they had an employee that was skimming at the window. It turned out it was his first day and they offered to reimburse me, which was nice. I am the person that hands back the extra money when a clerk gives me to much and I have no trouble saying the change was wrong. But ever so often we are in too big of a hurry and do not notice it until later. Fast food places hire a lot of teens, so if they have one looking to make more than it should, then the manager needs to be notified.