One of my favourite ways to save money on groceries is also one of the easiest - I check my bill. Then I check it again. I almost always end up finding a mistake in the store's favour.
I would estimate that about 75% of the time I am overcharged. I don't think it is some sort of dark, covert plan to increase corporate profits, just honest mistakes made by hard working cashiers.
Usually I steer my cart towards customer service and take a moment to go over my bill BEFORE I leave the store. The times I can't wait to get out of the store I check when I get home.
It has become a bit like a detective game, and playing Inspector helps me find pricey irregularities.
Regardless of whether I check right away, or come back later with a problem bill, the store is always 100% accommodating. Without fail the overcharge is refunded with a smile... so I can buy more food from them.
People make mistakes, but that doesn't mean that you should have to pay for them. Check your bill and save even more on your groceries.