I started using grocery self-checkouts during COVID, but I've kept using them because there's rarely a line (and I'm a misanthrope). I'd probably go back to using regular human checkouts if I had to dig through all my crap to prove what I bought.
Having said that, I've noticed myself making mistakes. I've accidentally failed to scan an item, and I've accidentally entered incorrect codes for produce. When I notice, I fix them, but I've probably missed a few.
I guess the easiest answer is for grocery chains to reinvest some of those windfall profits and hire more cashiers.
Theft is one thing and who knows what the numbers actually are for self checkout. Even with theft and us making a mistake or two, they don't have to pay cashiers, I'm sure they're coming out wayyyyy ahead.
I doubt they're coming out way ahead considering these cashier's usually get paid minimum wage, £10 and hour to not have people steal is probably more profitable.