Theft is more common than human error by a pretty big margin, but how big probably depends a lot on the store and the area. The store I was in was in a relatively upscale area and I still saw significantly more intentional theft than mistakes.
Plus human error goes both ways and kind of washes out. People do sometimes double scan and don't catch it, then overpay. The only thing theft evens out with is the money the companies are trying to save by employing fewer cashiers. It's a bad tradeoff, self checkouts are just all-around worse for everyone, and they should really go back to only having a few available for small purchases instead of being the primary means of checking out.
