Supermarkets these days have self service checkouts, it is part of cost savings, one member of staff can manage six tills. These machines are also adept at making extra money for the supermarket.

Yesterday I went to my local Sainsbury on the way home from work and used the self service tills. I had one of their many BOGOF (Buy one, get on free) offers but saw that when the machine asked for my card it was not showing any discount. I hit the back button and suddenly my discounts where applied.

Concerned I went to speak with customer services who said, very matter of fact, that it was a "known issue"! So next time you are using the self service tills double check that the discounts have been applied by going finished > back > finished before paying.