Perhaps fillet steak was a bit of an extreme example but for a large shop it could be fairly easy to miss being overcharged £20 or so. Lets say 10 packs of bacon instead.
I have no idea, probably just have to rely on a bit of goodwill from them. Obviously it isn't really much of an issue for 23p cans of kidney beans, but if was ten fillet steaks at £6/7 a pop then f**k knows what the protocol is
Ordinarily I just throw grocery receipts in the bin, but I was having a quick glance at mine there just now, and noticed that Lidl has charged me for 10 tins of Kidney beans instead of the 2 I bought this morning.