I write this post in comparison with what's tolerated in my own home-country and in the countries I've visited. Let me know in the comments what is acceptable in your country!
1. Don't open products and definitely don't eat them or even taste them before purchasing, even if you intend to purchase them and even if you are already on your way to purchase them.
2. Never taste a product and then put it back (i.e.; take a bite of bread or apple. True story- witnessed by me many times in my home-country.)
3. Don't eat the product while you are still inside the store, even if you have already paid for it.
4. If the Japanese supermarket near you got the "sashimi preparing corner", save yourself the embarrassment and don't ask them to let you try the taste. Unlike the similar corners of cheese and hams in foreign countries' supermarkets, in Japan sashimi and sushi are not given out for "tasting".
5. The reason for a microwave being placed near the cashiers is for you to be able to warm up the ready-to-eat meals. After you warm up the meal, it is intended that you eat it elsewhere and not camp out next to the microwave. *Some convenient stores got a seating area where you can eat your meal.
6. You cannot "assemble" your own fruit/veggie basket/package from the already prepared ones. Try to choose the one you like most without exchanging the content.
7. When at the fruit and veggie corner, when you see apples and the sign "100円" or "200円" next to it (approx. 1$, 2$) it means that each piece is sold for that price. Do not mistakenly think it is by the kilos. Japanese never sell by the kilo. Sometimes the produce will be sold by grams (for example, 100 gram of pumpkin for 100 yen), but most often it is by the piece.
8. Japanese supermarkets and food shops don't do the "1+1" or coupon sales where you cut out coupons from the newsletters. Instead, if you want to buy cheaply, come in the evening, about two hours before the closing. Japanese supermarkets and food shops discount all the food which is about to expire by up to 50% (50% being the food which expires on the day or the next day).
Also, supermarkets usually have a permeant corner filled with discounted products which are either expiring soon or are not popular.