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.

You can build a socioeconomic graph of the country by how anal the unexpected item in the bagging area sensors are.

Some places will detect a fly farting on the damn scale, others can take three or four kids climbing on it before it complains.

This is a more expensive form of shoplifting though, idk why even bother with the banana thing, as hilarious as it is.

Agreed, but there's nobody looking if you're putting the items in the bagging area or not. You could simply leave an item last, pay, put it in the bag, and go. They do have (prominent) cameras over the tills I've seen, though, not sure if that's just "we see you" or if they're doing some item recognition with that.