We can strong-arm their argument a bit. It's not true for me in particular because I buy a couple items every day or three instead of less frequently in a large shopping cart [0] and tend to buy items with known prices where I don't feel like I'm being scammed, but I still see errors every week or two, which should approximately correspond to an error every 2-week-cadence shopping trip. Common examples:

1. This isn't an issue where I currently live, but it's a problem in a subset of stores in maybe 1/10 cities I visit. Some stores routinely get state/local taxes blatantly wrong, even mega-chains like WalMart. They charge you one thing (coincidentally, never _less_ than the actual sales taxes owed) and remit a different amount to the governing body. These aren't small changes; it's charging an extra 1-3% on every transaction. The problem usually persists for years, I assume because the store isn't incentivized to fix it, enforcement is slow, the damage is low enough for each individual that lawsuits aren't worth it, and the damage takes long enough to build up that the overhead of a class action isn't worth it immediately either.

2. Much more frequently, stores get the tax wrong on "classes" of goods (e.g., the dividing line between tax-free grocery items and other foods). This is, perhaps, understandable given the number of unique items being sold, but it's very common for a grocery item to be inappropriately taxed.

3. The textual description of a sale is usually a bit different from how it's configured at the register. This matters more at stores like Safeway where the "sale" price just drops things down closer to the actual retail price of an item, and where there's a rotation of most items being on sale. Sometimes this is benign (a sale sticker doesn't have physical space for a lot of nuance), but some are more blatantly problematic. One example: Most sales don't have item caps (if a can of beans is on sale, I can buy both of them and have them both discounted), most exceptions are explicitly listed (e.g., "max 6 ears of corn per customer"), but sometimes things fall through the gaps, and the listed price will only apply to 1 out of the 3 things you purchased, leading to a surprise at the register. Another example: Required quantities are usually wrong. I remember one instance of needing to by "2lbs of chicken" of a particular variety, there only being one pack available (which happened to be 4lbs), and being told at the register that you had to buy 2 packs -- the fact that the sale couldn't be executed because they didn't stock enough was annoying (not common IME with chicken, more common at that store with other goods) but understandable, but the fact that the units were blatantly wrong and couldn't be discovered till you brought a refrigerated good to the register is a lot worse.

4. Mis-stocking causes a lot of confusion. Sometimes it's obvious which price goes where, so a careful shopper can avoid issues before the register, but sometimes the description attached to the price tag perfectly matches the item (and, if you search around, apparently several other items in the store) but is intended to refer to a different item. Barcodes don't lie though, and when you bring the wrong bottle of wine to the register you can get a nasty price shock.

5. Malicious rounding is pervasive. Last I checked (it's been a number of years), Amazon always rounds up on taxes as an example, which is an incorrect and illegal calculation in most US jurisdictions. Physical retailers behave similarly.

And so on. It's usually a matter of pennies. Sometimes it's a matter of $10-$100. Stores make many small mistakes, maybe maliciously, maybe not. Do with that information what you will.

[0] Not really relevant to the point at hand, but doing so fits better into my lifestyle. Stores aren't ever busy during my lunch break, by shopping so frequently I know instantly where everything is and can get through the store in minutes, my fruits and veggies are always fresh, it's easy to tailor meals to what I need to use up first, and for personal health I like to walk/bike 30-60min every day anyway with the shopping trip serving as an excuse.