The original barcode (which is still present as part of the discounted code) should allow the tills to look up the weight.

Problem is, every time it beeps at you and the cashier overrides it, it averages your item's reported weight with the ones its seen before. So if a cashier is overriding it all the time (as tends to happen with constantly beeping things) the weights drift off, and the whole thing is quite useless.

I was just going to chime in a with a similar comment.

I'd love to know if a friendly Tesco store manager would get in trouble for helping to run an experiment with this though. If head office got wind - or were informed by the manager - I'd presume they'd either go mad and threaten/fire the manage in question, or congratulate them. I couldn't predict it either way.

Someone testing the hack could buy two of the same item, one of which has the hacked barcode. The tester could then immediately point out the error, so no fraud actually occurs. A better way of doing this might be to use two people pretending to be a couple, with the first person checking out separately with the correct price. Then the second person could check out with the hacked barcode, then immediately point out the error.

This also gives a good control for the "experiment."

The current "modern" equivalent of swapping price labels is to buy expensive produce at the self-check, but to indicate you're purchasing low-cost produce.