That is something you can do in cahoots with a regular cashier and the reason places like Costco check your receipt. The cashier just has to fake scan an item, and nobody would notice. Receipt checking makes it possible to get caught.

Most self-checkouts I've come across have weight validation – "Unexpected item in the bagging area".

Categorising things as "bananas" tricks the checkout into accepting the weight of an item, and you pay the appropriate price per bananagram.