Use this advice for items that you see that you couldn't find everywhere. For example, a good birthday gift for your children's friends, or party favors. You need to shop the unit price to see if you're getting a bargain. Use this strategy especially for cars. Look over your checks.
Waiters make mistakes.
ATM's make mistakes. The IRS makes mistakes. Do phone-rate checkups. Watch your interest rates. Don't waste money on food.