We had one of these signs posted in electronics at Superstore when I worked there. I had a customer purchase a TV that rang in $30 more than what the label read, and he pointed the sign out to me. Of course, I was confused.
Of course, he wasn't going to get a $247 TV for free, so we sold it to him for the price that was on the label. He was fine with that. The point is, that sign was up there for I don't know how long, and I was completely unaware of it.