I've twice accidentally left without paying (or paying in full), but never unwittingly (unless, I suppose, I never realised my mistake at all and hence still don't know about it). On both occasions I attempted to pay, found I was short, and went back and paid later.

Oh, and there was one occasion on which I took something from a shop without paying, with the consent of the shop: Sainsburys had a pack of cheese with a French price label, which confused the till assistant so much that their supervisor decided it was easier to just let me have it for free.