The bug

When you have a world with seed zero, i.e. when

/seed

outputs 0, then it cannot be recreated using the "Re-Create" option, because it will be interpreted as a random seed. To be clear, I know that putting "0" in the seed option is supposed to create a random world. However, I would expect that copying an existing world with seed 0 would also generate a world with seed 0.

How to reproduce

As mentioned, putting '0' in the seed option on the "Create New World" screen will be interpreted as a random seed (Personally I find this unintuitive behavior, but this has been resolved as intended: MC-1820). However, a world with the seed 0 can be generated either through external tools, or by choosing the seed text very carefully. Here are some options for the text, that will generate a world with seed 0:

tainsntoh
f5a5a608
af4e962cd