First of all, BOGO sales make me crazy. I don't want two of something at buy one get one at 50% off, but I likely would have bought one at 25% off.
Secondly, I really think it is up to the consumer to read their coupons and know what the rules are. The cashiers have no control if your coupon doesn't match the criteria when you get to the check out.
I use an APP called FLiPP to do all my grocery shopping. It takes all the local grocery stores, looks at your shopping list and finds the cheapest in the area. Then, it makes a list of those so you can go to your favourite store that price matches and just price match everything, showing them on your phone the ad from the other store. BUT you have to make sure you are buying the exact same thing. I've seen so many people argue with the cashier...I feel bad for them, but it isn't the cashiers fault that you haven't fulfilled the coupon's details.
Watson