I was in Tesco's recently and spent about £30. However, I forgot to use my Tesco vouchers which were worth a few quid and extra clubcard points. I said to customer services what had happened, but as it was nearly midnight, I was advised to come back the following day and it would get sorted then. I returned the next day and got £5 in cash and 150 clubcard points too without any hassle from the customer Service staff.
If you ever forget to use vouchers, keep your reciept and Tesco will honour them the next time you go to Tesco (although you'll have to try to remember to do this should you forget first the time).