Right I don't know if anybody has mentioned this before but here goes..
Shopping in tesco today (Sunday) and supposingly they were having a HUGE problem with products not being discounted when they should be. Bought some face washes at 2 for £5, both £5 each anyway.
When getting a refund they doubled the difference of what I've been overcharged, making them basically pay me to go out with them! So it makes it worth it to check you got the discounts you were ment to!