Bananas that are frozen correctly should not go bad within 2-3 months. However, if your banana was rotten or if your freezer isn't working properly or is not at the correct temperature, then frozen bananas can go bad. A frozen banana that has gone bad will be black in color, will smell bad, and will be very mushy. Throw away the whole bag of frozen bananas if even one has gone bad, because there is a chance that mold may have gotten to all of the frozen bananas in that particular bag.