Freeciv

Zero as random seed means that the seed is not preset, but is to be got from a random source. With really bad luck, some random source types can result in also the final seed being zero. When that happens, it makes it impossible to replay the game by setting the random seed (as setting seed to zero would cause the game to get a new seed from random source), and likely cause other problems.
Likely similar thing can happen with map seed.