Don't blame the stockboy. He's really not getting anything out of taking down the sign, the bosses probably told him to do that. It doesn't benefit him at all.
I had some crap pulled on me at safeway over advil. The girl looks at me and goes oh, it's an old sign. And I just about blurted out "And...?" because seriously wtf. Who cares, you guys left it up. That is your bad and it doesn't matter if there's an expiry on it, that isn't even the customer's responsibility. It's a displayed price.
Where I work, if an employee is taking a sign down when a price check is called for, it's because they're bringing the expired sign to us to use as a confirmation and then to get rid of, or if it's a current sign, to give to the people who handle pricing to fix the error.
I think I've spoiled myself working where I do. We treat customers with the utmost honesty, respect and integrity, and I kind of expect to be treated the same when I go elsewhere. We don't BS people, and I expect the same and unfortunately don't get it most places.