When I was a kid and groceries weren't computerized with bar codes, the cashiers frequently made mistakes ringing up our purchases. I remember my parents checking off every item against the sales receipt. Things are a bit better now but still it's a good skill to teach kids to be aware of purchase prices.
Did they charge me the right price? What will be the price with tax? Did I get the right change? Did they do the foreign exchange correctly?
Here's an interesting and related item. There are many retailers in Canada that have adopted "The Scanner Price Accuracy Voluntary Code". One part of the code says "....where the scanned price of the product at checkout is higher than the price displayed in the store, the lower price will be honoured and ...if the correct price is $10 or less, the retailer will give the product to the customer free of charge or...higher than $10, the retailer will give a discount of $10 off the corrected price...". See www.retailcouncil.org for more details.
James Whelan, www.moneymatter4life.ca