In that scenario it seems to me that the best the thief can do is to "accidentally" ring up a steak as bananas occasionally and hope that if someone ever takes note that the past events will remain undetected.
That said, I'm pretty sure all of the self checkouts I've interacted with over the past several years would automatically flag such a "mistake". There are some things they're still bad at and they generate plenty of false positives but they seem to be reasonably good at identifying obvious "errors".