I'll tell you what is wrong with "banjo's", GUEST:guest. There is no acceptable plural. Banjos, banjoes and banjo's all have their devotess, but nothing seems to really work. On the analogy with potatoes, I personally go for banjoes, but opinions differ. Musically, of course, there is nothing wrong with them: if there were, why would God have decreed that they are the instruments angels play in heaven?