Cashier doesn't ring up an item and you don't find out until you return home, maybe you don't even find out for a few days because you don't look at the receipt. What do you do? If you take it back and try to pay for it, does the cashier get in trouble? Would it being a $2 item vs. being a $75 item affect your decision?
It depends on the store I guess. I've had that happen on occasion and I can't speak for where you live but that kind of thing is built into most stores insurance and overheads, at least over here, mistakes happen and sometimes an item isn't scanned or checked when they put them in your bag.
If it had happened with an item around $75 I probably would want to go back and say hey you made a mistake but if it was a $5 item or less I wouldn't usually bother and the one or two times it has happened it's only been something small like small chocolate or something under $5.