[autoavia]

No stock system is perfect. It is possible that the website showed one of something in stock and it got sold in the shop after the website order was placed but before the order was picked.
I know some shops have scanners at the till which automatically deduct stock as it is sold but not everyone can afford that technology and I have known shops where shop purchases were deducted from stock held at the end of each day.

 The other common reason for similar stock errors is that someone is picking a mail order and finds a stock item which is faulty so they pick another to replace it but the faulty item doesn't get deducted from the stock total.

 I also know of one local shop where shop sales were invoiced manually through the computer system at point of sale. Occasionally stock had been input into the computer under two reference numbers, often where the manufacturer had changed a ref number at some point. If the staff member found the now disused ref number which was showing nil stock they simply increased the stock on the computer to allow them to generate the invoice for the customer who was standing waiting at the counter which is fine but then the stock is still showing as available under the new ref number so the stock levels go haywire.  

 Computer controlled stock systems are far from perfect because somewhere along the line there is always room for human error.