This one seems intuitive when you read it, but I am surprised at how many people do not pay attention to expiry dates.
No point getting a gallon of milk if it is going to turn sour with a couple of days.
Same goes for meat, eggs, yogurt, spreads, frozen items, deli/bakery items etc.
Some people say you can use a few items a few days after expiry – but I personally value my health more than money and would rather avoid buying such items in the first place.