When purchasing sake in a Japanese grocery store, read the label carefully, if you have any ability to read Japanese at all. If, for instance, the English label on the shelf reads “nigori”, check the Japanese label to make sure that it isn’t actually namazake (生酒).
Why? Because while most good sake should be served slightly chilled, namazake must be kept in the fridge right up until the moment you’re ready to drink it. It’s not pasteurized, and if it gets warm for even a few hours, the live yeasts turn it into basically-undrinkable carbonated mush.