Our stock levels are constantly updated during the day.
During busy days (Especially at semester start) the same product can be purchased by several people within minutes, both online and in our 65 stores.
While it might have been in stock when you started the purchase, it could have sold out before you got a chance to complete the order.
Sometimes the product is still in stock, although we write that its "Out of Stock" on the website, as these Stock buffers are put in place to ensure we don't sell over the inventory count. Please contact Customer Care if you would like us to have a look for you.
It's also possible we made a human error with inventory count. It happens. Although we do our best to ensure that it doesn't.