The number one mistake, in my humble opinion, is buying something you don't use just because you have a coupon.
It's not saving you money if you are spending money on an item that you don't use!
Your real savings come when you use a coupon on an item that you do use in your household!
You get a coupon for Fruit Roll Ups. You don't buy them because you prefer your kids to have a real piece of fruit with their lunches, not a pre packaged (etc) snack. But, you have a 50 cents off coupon, and they are on sale this week for $1.47. You get the box for 93 cents. Are you really getting a good deal if you are compromising your shopping list and nutrition of your kids, if you purchase this item? Would you be better off buying the fruit that you normally purchase (and it happens to cost more this week because you are comparing the sale of the Fruit Roll Ups versus the fresh apples during the mid-winter)... it's tempting. But don't give in. You'll feel better in the end knowing that you stuck through and sent the kids with something you feel good about.
(Nothing against Fruit Roll Ups, it was just an example!)
The same could be said of sugary cereals, pre packaged meals, meal starters, etc.