Woolworths wouldn't have let me off the hook!

I bought a few items from the Woolworths store and when I came home I realised that I only had 6 of the 7 items I had selected but my slip only reflected 5 items. If the security at the door had checked the slip do you think they would've believed that the cashier had not done what she was supposed to do? Or what if the alarm had been triggered by the unscaned item? I could've been arrested for theft when it was the cashier's negligence. I am very upset by this as I could've been in jail today.