On Wednesday I was driving back from Oxford and dropped off at Tesco to buy some food. They had an offer ‘5 for 4’ — buy any 5 items of packaged fruit or vegetables and get the cheapest of them for free. I bought seven items. I would have expected to get the fifth cheapest item free, but their computer instead gave me the seventh cheapest item. Here is the evidence.

A few years ago, it was common for website designers to make errors in logic that enabled customers to get unanticipated discounts. These were seen as ‘security failures’. Nowadays it seems that programmers err on the other side. Thankfully, this has stopped the security problems.

Or has it? Here’s how to attack Tesco if you don’t like them. Go and buy six packs of fruit and veg, then take the receipt to your local Trading Standards and make a formal complaint. If a hundred people do that, it’ll cost them plenty.

The Internet allows the rapid dissemination, and anonymous exploitation, of vulnerability information, as Microsoft has learned over the last five years. Maybe there are variants of this lesson that will be even more widely learned.