I would absolutely say something if I got something I didn't pay for. If they made an error and left something out of my order, I'd certainly point that out so, why not the other way around? It might be a big corporation but, it's still an individual you're dealing with. And they might suffer some repurcussions from the error.

My limit is once. If you want to argue with me, it's not worth my time. The argument has happened many times. The worst is with one of my favorite bartenders. I can never tell if she's being nice and "forgetting drinks" which I doubt or if she's really forgetting to charge me for a beer here or there. I generally just add it to the tip or at least make sure the tip is enough to cover that beer.