OK, so IWBU to call her a tit but here's why. 
Went to Tesco - had £22ish until Tuesday and a voucher to save £4 if spent over £20 and £3 clubcard points. So add up as I buy. Came to £21.34 but some reduced items reduced further at time of scanning so scanned at £17 something - cashier let me go 3m to freezer section and grab some boxes of ice creams. (was about 10 seconds)
Cashier rang them through but 1 didn't register and the amount came to £19 something. I realised this was wrong and quieried it to the puzzled cashier who then pressed something and till registered I'd paid £20 - not my fault. And to be fair not really cashiers and she was as puzzled as me!
I looked at printed receipt and that's when I noticed the missing item and told cashier so she called for assistance.
Customer waiting started shouting at me that I should have added up shopping first - I did (explained about reduced further with apology).
Customer started shouting at me again stating 'she had plans that afternoon and didn't have all day to wait for me' I mentioned I had plans too but always plan enough time in case things at supermarket take longer. Any way she continued being rude to me - can't remember exactly what she said.
So we're waiting for assistance (about 2 minutes) and she starts shouting at me again. Ending up telling me I had held her 15 minutes now and I should sort myself out. My DS - bless him gave her the 
look, and I told her to
'stop being a tit and to buy a new watch as hers obviously didn't keep good time' 
Not my finest moment but at least my voice was quiet!
Great Customer service from tesco who charged me £16 and gave me the unscanned ice cream box for free - due to my inconvenience and the fact they couldn't fathom out to to correct the mistake. 