Check your till receipt

KosmicUnicorn

This happened to me yesterday.  When I told a friend she said it was common.  I didn't know about it so thought I would share.
I popped into a well known supermarket yesterday just to buy a few items of food.  My bill came to around £40.00.  As I walked away I decided to look at the till receipt as I got the feeling that I had not spent that much.  On the receipt was a pizza costing £4.34.  I had not purchased a pizza!  I emptied my shopping bag.  No pizza.  I counted the number of items on the receipt and there were 14.  Number of items in my basket 13!  
I made my way to the customer service desk and told the lady what had happened.  She took the receipt and I was able to show her the mistake.  Together we counted the items and I had 13 against the 14 on the receipt.  I suggested she tick off the items on the receipt as I told her what I had.  Again, the pizza was the odd one out.  She did say that there was no problem she would just refund the cost of the pizza back onto my card.  She used her scanner to scan the barcode at the bottom of the receipt.  It would not scan.  She tried a couple of times.  Still would not scan.  She then tried to key it in manually.  The barcode did not work on their till system.  Not sure how but she did manage to refund to my card.  She did mention that their tills had been down earlier.  I just don't understand how the till being down would make an item appear on my receipt.  I also know that the lady on the checkout didn't scan a mystery pizza.  She had taken over the till as I was about to be served.  She said that she couldn't hear the scanner so turned up the volume of the till.  Each time she scanned the bleep was loud.  I was also packing as she was scanning so I would have noticed a double bleep.  She did in fact do a double scan with my first item - dog food - which she removed from the receipt.  That was clearly shown so was accurate.
Has this happened to anyone else?
Does anyone know what could have caused it?