Today, implementing a custom __hash__() for a class, I had to deal with the fact that in many #Python implementations hash(-1) is equal to hash(-2), both are -2, and so are the hashes of (-1, -1, -1, -1) and (-2, -2, -2, -2). The good news is that I knew about hash collisions and could quickly figure out the problem and manage a workaround :)