This Guardian news story describes how Tesco have been overcharging customers.

An undercover reporter was overcharged on multi-buy offers at most of the stores (33 out of 50) visited, because marked promotions were out of date and no longer valid at the tills. Tesco has now said that it will check the prices of every item at its 3,500-plus stores across the UK, which is a major task.

This kind of problem is not uncommon. I had a very similar issue recently at Netto (a discount supermarket in Germany): they have a promotion on boxes of 6 bottles of several wines, and I bought one last week, and was charged the per-bottle price, not the multi-buy price, due to a mistake by the check-out clerk.

The law, however, is quite clear on this, all across Europe: shops must sell you goods at the advertised price, whether it is advertised in a promotion brochure, on-line, in-store, or on the shelf or product itself. If you don't want to be overcharged, you need to check the price that the check-out clerk rings up, and if it is wrong, complain. In these days of bar-code check-outs, if the price is actually wrong in the check-out system, this can create quite a problem at the check-out, involving a supervisor being called, but you just need to insist.

Many people don't like to complain, but sometimes it needs to be done.