Any couponer will tell you, Print The Store Coupon Policy! Carry it with you when you shop. Make sure its updated. I check the store website every 3 months to make sure the policy has not changed.
You will have people and checkout machines tell you that you are wrong. This is why having each store's coupon policy is so important. I also write down the contact number for corporate. Because the sad truth is, some store employees and manager's do not like couponers. And when they believe they are right, they will take a stand. You have to put your game face on. And equip yourself with the right tools to win.
Some steps to avoid problems is to always be friendly. I greet store staff all the time, and I have a few cashiers they I am on very friendly terms with. I also try not to shop at peak times. I love my all night Giant, because on Saturday I can head over after the kids are asleep and shop away.
I guess my worst experience with couponing so far happened at Safeway. They had a an awesome sale on hot dogs. And if I combined the store coupon and the manufactor coupon, I actually would earn .1 cents per pack of hot dogs. Their store policy says they I cannot actually get cash back. But the coupon can be entered manually the even the price and the discount. When the manager there told me I could not do this. He said I could not earn money back when my order was scanned, and then my coupons started a beeping frenzy. I explained to him I did not want the money back, and that the coupon discount could be adjusted. I even showed him the store policy. But he was adament that I was wrong, and we went back in forth a good 5 minutes. I finally asked for my entire order to be voided, and walked out steaming mad. I called corporate and sent emails, and I did end up with a $10 gift card. But at the time I was embarrassed. This was my local Safeway, and the manager made me out to be a true crazy couponer who was trying to cheat the store somehow. So sometimes even having the policy doesn't not equal a happy conclusion. It happens. But knowing the policy, and becoming familiar with it and the store's staff is a great way to ensure it doens't happen often.
Safeway http://www.safeway.com/ifl/grocery/coupons-policy
Giant Food http://www.giantfoodstores.com/shareddev/sharedcontent/Savings/DoubleandInternetCouponPolicy.cfm
Target http://www.target.com/Coupon-Policy-Store-Merchandise-Information/b?ie=UTF8&node=2223357011
Wal-Mart http://walmartstores.com/PrintContent.aspx?id=7655
Walgreens http://www.walgreens.com/topic/generalhelp/coupon_policy_main.jsp
CVS http://couponsenseblog.com/cvs/cvs-coupon-policy
Rite Aid http://content.riteaid.com/stores/CustomerCouponAcceptancePolicyAugust2010.pdf
K-Mart http://thekrazycouponlady.com/2009/07/09/kmart-coupon-policy/
Albertson's http://www.cuckooforcoupondeals.com/questions/coupon-policies/albertsons-coupon-policies/
Shopper's http://www.weusecoupons.com/upload/shoppers/25835-shoppers-coupon-policy.html
Kroger http://www.kroger.com/mykroger/034/Pages/houston_announcement.aspx
Food Lion http://www.foodlion.com/assets/docs/customerService/CouponPolicy_Food%20Lion.pdf
Whole Foods http://forum.moneysavingqueen.com/showthread.php?14086-Whole-Foods-Official-Coupon-Policy
Harris Teeter http://www.harristeeter.com/promotions/online_coupons/coupon_policy.aspx
