Check your receipt
Checking your receipt after you buy something is an excellent habit to get into, especially when you shop at stores that have pricing guarantees. As you shop, make a note (mental or otherwise) of the prices listed on items, especially if they are on sale. Then check your receipt before you leave the store to be sure you were charged the correct price for each item, and that any discounts were applied. In addition to not losing money by being overcharged, you can sometimes actually get items for free by doing this (depending on the store’s policy). Some stores will refund you the entire purchase price for items up to a certain amount if they overcharge you. It’s worth it to take a minute to make sure everything is correct.
This also applies to bank receipts. When I deposit money, I always go online and make sure the deposit posted. One time, I did this and didn’t see the amount added. I checked my receipt and 2 of the #s int he account were transposed. I went straight back to the branch and got it corrected. Now, I check the #s as soon as I get the receipt from them.