That's when I get into the habit of photographing price labels while I put items in my cart, and upon finding a discrepancy, abandoning the entire cart full of goods for an employee to return. Make it costly for the company.
I've already got my phone out, because that's where I stick my shopping list sticky note. I already watch prices and frequently catch mistakes at the till. But I trust that the paper price tag won't change, so I just tell the cashier their system overcharged me and they fix it. Occasionally I'm hit with an unexpectedly large bill and spend an extra 30 seconds or so to figure out what's going on (the answer is usually cheese).
But if they change to electronic price tags, you bet your ass I'm gonna spend that extra few seconds per item so I have evidence on hand.