I have to say that since I wouldn't have been totting it up as I went round, I'd probably just have been pleasantly surprised at the checkout, and if I'd checked once I got home, whether I went back or not would depend on how far/easy it was, how tired I felt, and how much else I had to do.
I did once go back and pay for something the assistant hadn't charged for. But on another occasion, when I had Gdd in the buggy - dd busy with new baby - and was doing some shopping, mostly in a basket slung over the buggy handle, I got to the checkout thinking I was going mad. I could have sworn I'd picked up 2 packs of the expensive cat food - the only one their cat will eat! but there was only one.
Still, I paid and took Gdd and shopping home - only to find that I'd evidently put cat food pack no. 2 on the buggy hood, since basket was getting full, and it had slipped right down between the folds.
I went hot and cold to think how easily I could have been accused of shoplifting - who on earth would have believed me?
But I'm afraid didn't go back and pay for it - it was getting late, plus it was a fair old walk, and starting to rain, and dd wanted help with tea and bath time, after which I was cooking the dinner, followed by a one and a half hour drive home.
I still feel bad about it, though!