Customers can be forgetful too. Yesterday a customer came to our shop and told us that she only received a bottle of the item instead of two which the receipt shown.
With the receipt provided, it's easier for me to check the cctv. So I had to really check through and found out that those two bottles were indeed put inside the plastic bag. That confirmed that she had taken them.
That bottle could be somewhere at her home as someone could had taken it out from the bag. That shows that they could be absent-minded but what I could do is to confirm the reality.