Supermarket "sales": Whether you're a SuperValu (SVU - Get Report), Kroger (KR - Get Report), Safeway (SWY - Get Report) or A&P (GAP) customer, you've likely faced the same scenario: A shelf at the end of the aisle with four of your favorite products on sale for $10. As you regularly pay $3.49 for this foodstuff you can't live without, you're overjoyed at the possibility of acquiring four of said item at a 28% discount. What you're not looking for is either the missing part of the sign that says it's $2.50 no matter what, or the part that tells you it's $2.69, still a better deal than that $3.49, but not as good as initially advertised.
This one isn't the market's fault as much as it is the consumer's; it's human nature to stock up in the interest of frugality, hedging against, for instance, commodities price hikes that raise the cost of coffee. But when either the perishable item goes bad or the nonperishable goes forgotten in the cabinet, it's not the store's fault you need a new one. The easiest supermarket rule is the one that prevents most wide-eyed buying problems: Stick to the list.