Especially when it's in the sale?
Now, my understanding is that as a consumer you cannot make a retailer charge you the price an item is listed at in the shop, however, a retailer could get into trouble for false advertising / marketing if they are selling things for a different price than they are displayed as.
I think it is very poor customer service to get to a till and find that you are expected to pay an extra £5, £10 or whatever than the item is priced as.
Obviously I don't mean that I should be able to pay 99p for something that clearly costs £99, but....