Mistakes happen. If a business is consistently marking things lower, in some attempt to get people to bring it to a counter and hopefully get more money out of them that's obviously morally wrong. Giant corporations with advanced systems for managing all this crap get less empathy from me.
If a small business just accidentally had the wrong tag on an item, or said tag was incorrect I think basic human decency would just have people accept that "oh the business made a mistake". But everyone seems to somehow make themselves feel personally slighted because they worked themselves up into a giddy that they were about to get away with getting an item at a lower price, along with their walk to the counter and the minutes of crucial time invested only to be told "No."
It's like those threads where an item is very clearly priced incorrectly on a website and everyone piles on to order it, when it is very clearly incorrect, then throw a fit when their order is cancelled. A business should be no more obligated to sell to you, than you to buy from them, and that works all the way up until money is transferred for goods or services.
Maybe I'm just jaded, maybe it's years of the ridiculous phrase "the customer is always right", or maybe the prominence of Mega-Corporations constantly attempting to screw over the consumer has really led to this mentality, but everyone walks around with a chip on their shoulder and like they are owed something.
That being said, Customer Service is never a bad thing, and provided they aren't going to lose their ass on honoring the price, it isn't the worst idea to at least consider it.