Mr. Hutton says they were overcharged on a regular basis.

"It all came to a head when it was happening every week. We went something like six or seven weeks and every week there was something wrong," Mr. Hutton said.

"They charged us twice for - yoghurt it was actually - and I'd only had one tub of yoghurt but I'd been charged for two. So whether that's the scanner's fault or whoever's operating the scanner, it's hard to say," Mrs. Hutton said.

American Tim Duffy is no ordinary shopper. When someone says the word "supermarket" he thinks, "free groceries".

Mr. Duffy claims to have made thousands of dollars from supermarkets overcharging, because of their policy of giving you the goods free if you find a mistake.

"They've called me the 'scanner commando', the 'price-check champ'," he said.

"I don't get mad, I get even and I just started to learn the system, learned how to read the codes when they made their price changes and during the next year I got $1,800 out of that one store in free merchandise."

Mr. Duffy is that good at finding incorrectly-priced groceries he's hit the headlines in the US and he now receives $100 of free groceries every time he shops.

Understandably, store managers cringe when he enters.

"I would walk up with a basket and say they that 'these are mis-priced' and they just say, 'take it and leave'. They didn't even want to scan it," he said.

Barry Urquhart is a marketing analyst and supermarket watchdog.

He says overcharging at the cash register is costing us millions.

"In many instances consumers aren't aware that they are inadvertently paying more than they thought they would be," he said.

"World-best practice would say for every million transactions, the most that this should occur is four times. It's certainly occurring more than four times in every million transactions."

It is estimated that in Australia, one in 100 customers are charged incorrectly.

"They're not looking for it and therefore if there is a price variation it goes through and they pay the difference," Mr. Urquhart said.

Stan Moore, from the Australian Retailers' Association, says shoppers should have every confidence in our supermarket scanners.

"Our figures would indicate that there is an underlying, very, very small level of machinery error and the balance probably is down to human error," he said.

"Our mistake rate is probably around about one in 1,000 items. That's something we're working on but clearly that is a very, very small risk ... so at the end of the day, the consumer is probably getting a fair deal."

We decided to put the "scanner commando" to the test.

He received freebies at each of the three supermarkets we visited.

"They're priced $2.19. They're supposed to be on sale for $1.99 and they're scanning the $2.19 so we got it for free," Mr. DuffY said.

"These both were listed at $4.39, they're actually scanning $4.99."

So here's a few supermarket tips from the "scanner commando".

• Take the specials catalogues with you when shopping to compare prices.
• Mark down the shelf price then after you've paid at the cash register compare the shelf price with the docket price.
• If you've been ripped off, stores have different policies. At Aldi you'll only receive the overcharged amount but at Coles, BI-LO, Woolworth's and Franklins, you'll receive the overpriced item for free.
• Whether you're buying one item or a dozen, write the price down no matter what store it's in because you may find errors.

But remember, at the four major supermarkets, if you buy more than one of the same product and they're overpriced you'll receive the first for free and the difference back from the other products.

"Be aware it does happen. It happens all too often and you are going to pay," Mr. Urquhart said.