Industry reports consistently state that around 80% of all retail staff fraud is conducted at the point of sales by till manipulation. Till manipulation fraud is the alteration of transactions on the till for the purpose of liberating cash or goods.
There are many classic methods that are used. Full details can be found here.
Void last / void line item / error correction
All tills allow the operator to correct any mistake during the transaction.
During a sales, the operator will ring up a number of items from a real customer, conclued the purchase with the customer, but without finalizing the transaction. The operator will then void one or more line items from the transaction and finalize the transaction at the lesser amount, pocketing the difference.