if there is a discrepancy between the advertised price, the sticker price, the scanner price or the display price and the checkout price on any grocery item, a food store or a food department shall charge a consumer the lowest price. If the checkout price or scanner price is not the lowest price or does not reflect any qualifying discount, the seller:

• shall not charge the consumer for 1 unit of the grocery item, if the lowest price is $10 or less;

• shall charge the consumer the lowest price less $10 for 1 unit of the grocery item, if the lowest price is more than $10

  — MGL I.XV.94.184.C

The grocery store is required to put a sign at each register describing the law, which means that when you notice this you can point to the sign. Which is way better than trying to show the cashier the relevant text of the law on your phone would be.

I have fun trying to remember the price I see for each item as I put it into my cart so if it rings up at a different price I can point that out. The law has an exception for cases where the price is a "gross error" (off by half) but in most cases discrepancies are small: ringing up at $4.99 when it said $4.50 on the shelf. Because you get the item for free if they've overcharged you, however, what matters is just that they put a misleadingly low price on the shelf.

I've noticed stores rarely have a good system in place for fixing these problems. When I catch one they generally check and give me the item for free, but that doesn't usually translate into fixing the price on the shelf. Which means that when I come in next time, it's often still wrong.

This seems like something that a group of shoppers could use together. Whenever anyone noticed a mispricing they could post to a mailing list ("the store brand blueberries are marked $3.99 but ring up as $4.29"), and then everyone on the list could go get some free blueberries. This would probably get stores to be faster about updating their prices.

Even if the stores got very fast at fixing things, though, it could still be rough for them. Say one person goes through and notices they've been overcharged for something. They don't say anything to the store, but instead write to the list and name a time. At the designated time a group of shoppers pick up one unit each and fan out over the store's checkout lines. The items are all scanned, the shoppers all object, and the store has to give away one item per checkout line instead of just one item total. This could be a parody heist plotline in a sitcom.

(While this is hard to fix with technical means, if people started doing it, of course, they would update the law.)