A proverb that I heard as a child, but I don't know its origin. Maybe I even invented it, I don't remember. But I am sure that you will either find it or know it, or you will invent one based on the image.
The proverb refers to those who focus on the part that is not relevant to the detriment of the one that really matters. Clues: tail, bite.
Version totalement hors sujet et légèrement édulcorée, de Pierre Desproges, mais j'assume !
It is not the dog's tail that will bite you.
Ce n'est pas la queue du chien qui va te mordre.