Had my hands full with some stuff I was carrying at the time so couldn't be bothered to stop and pick it up, especially since I was in a tiny bit of a rush. As for finding out at the register, I know what you mean, terrible feeling, part of why I always use the calculator on my phone when shopping to make sure I never go over (also so they don't rip me off by not counting discounts or shit)
The shelf prices DO lie though, that's the thing. Sometimes an item will say "X% discount" but when they ring it up it doesn't count the discount. Had this happen fairly often in some less-reliable stores, so just got the habit of checking prices myself on a calculator, since even a 1€ discrepancy is too much to ignore when my entire month's budget is like 200€...
If shelf price is 1.99€ and register price is 2.49€, you won't really realize it, especially if you buy a ton of stuff, see my point? If I knew it would be charged more expensive than on the tag, I could have chosen a different product instead, should it ever happen.
Good for you, I just can't be bothered, thus the calculator Sometimes it has saved me from a lot of rip-offs such as one time I got some biscuits that were supposedly 1+1 yet was charged for both individually then made a ruckus about it at the register since they insisted the tag on the shelf was wrong and I had to pay for both even though the law here states that I'm required to pay the TAG prices.
I know how it is in the US, hardly anybody compare so they rip you off a couple cents at a time... As I said about how it is here, the law specifically states that the customer is required to pay what the shelf price is supposed to be rather than what comes up when they ring up the items. I've had FOUR times where they tried to charge me like 3€ even on single products yet was saved because of that habit I mentioned
What I mean is that I've come across products suddenly being higher priced when I get to the register. But I think part of that is to blame on the digital tag prices, that let them change the prices literally whenever they please. Sometimes, they might change the price between you grabbing a product and paying for it. Hasn't happened to me, but has happened to friends of mine.