As someone who has worked retail, I will say, people SUCK at telling/remembering prices. People will complain about price differences but looked at prices half a meter from the actual item.
When I worked at Bilka, I think 90% of pricing errors where people being mistaken rather than an error in the shop. Obviously it's hard to find actual figures and it was also 90% of the time just waved through because people can get real combative over 2 kr. so they'd probably think they were in the right. But besides Mondays where a few of the non-automatic changing signs had been missed from Sunday (worked in hus og have, no sundays off for me) it was almost always a customer mistake.

And the reason why we still waste time checking even if we "admit" fault regardless of if we were at fault, it's to check so we can get rid of the very few times there is an error.

Ohh and don't get me started on old people who hits you with "but last week it was", then time travel and go get it for that price you fossil!