Charging for items not purchased

My mother and i both shopped at Woolworths The Square Northmead today. My mother discovered an additional charge on her till slip for R66.99 for an item she did not purchased. On contacting me i checked my till slip and discovered an additional charge of R66.99 for an item i did not purchase. We have still to go back to the store to complain, however i do recall a similar complaint on FB. This is unacceptable, and if this is a systems error i would expect Woolworths to warn their clients. If this is not a systems error then i am completely at a loss and expect an explanation.