In case you missed it: This was one of our most-read stories of 2017
When you go shopping, do you casually throw your receipt in with the groceries, neglecting to read it over?
Or, worse, do you decline to print one out and walk away, ignorant what you've paid for all those carefully-selected items?
If you answered "yes", you could be ripping yourself off, with scanning errors causing supermarkets to overcharge for everyday items - and many shoppers failing to detect, or act upon the mistakes to recoup what they are owed.
A survey of 2,141 Australians by comparison site finder.com.au found that two in five people had been overcharged at the till in the past year.
But one-quarter said they didn't bother checking their dockets and, of those who did, they would only bother going for a refund if they were overcharged by more than $10.
Finder's money expert Bessie Hassan said it was up to shoppers to spot mistakes and challenge errors.
"You might be left out of pocket and not even realise," she said.
"An advertised discount might fail to be applied or an item might scan twice - whatever the reason, shoppers get overcharged all the time."
A payment processing glitch caused Woolworths customers to be charged double for their purchases earlier this morning, prompting the retailer to apologise and reverse the transactions.
While the mistake was not with the retailer in this case, scanning errors are a more common than some may realise - and shoppers may be entitled to more than just a partial refund.
"It does happen," the Australian Retail Association's Russell Zimmerman told news.com.au.
"It's been going on for a long time and it's very easy to have mistakes when you've got the amount of skews that a major supermarket has got."
Keeping barcodes up-to-date with price changes was part of the incredibly complex task of running supermarkets that stocked thousands of products, he said.
THE LITTLE-KNOWN FREEBIE PROMISE
What some customers may not know, is that if an item scans incorrectly, they may be entitled to pocket it for free.
That's the rule enshrined in the Scanning Code of Practice, which is now administered by the ACA.
Woolworths is the only major supermarket that is currently a signatory to the code, while Coles and Aldi each has its own version.
These supermarkets promise that if an item scans at a higher price than that displayed on the shelf, the customer will receive it free of charge.
If buying multiples of the product, the first item will be free and the rest will be charged at the shelf displayed price.
Exceptions include Aldi's Special Buy items, for which customers will be refunded the difference between the shelf price and scanned price, the discount chain said in a statement.
Coles' policy does not apply to alcohol or tobacco products, third party gift cards, items without a barcode or those worth more than $50, its website says. For multi-buy offers such as two for $3, the offer will be honoured, with no free item offered.