On another site, a third hand story was told: In an office supply store, expensive printers go missing without payment. They find out that an employee is so obsessed with selling extended warranties that when she sells a printer and manages to sell extended warranty as well, she fills out all the forms, charges for the warranty - and forgets charging for the printer.
It seems that this is indeed due to stupidity and not intentional. Can she still be guilty of theft? And if a customer walks away, having paid exactly the amount the cashier asked them for, but without having paid for the printer, could that be theft? If not, do they owe the money for the printer?