I used to be a cashier. This happened more times than I wanted it to and usually happened when there were no associates on the floor to help people find things. At clothing stores, customers actually want you to hold their clothes while they shop for more clothes. Uh, no. I have limited space on this counter, okay? The bottom line is, PLEASE DO NOT check out until you are 100% ready to check out. And my God, if you have to pay 50 cents extra for something, it is not the end of the world, not worth a price check, and not worth holding up a line to go check a misapplied tag. All cashiers everywhere thank you.