After making several copy's of a survival world, deleting and renaming them (in the nether) vertical "walls" started appearing in the world. (I sometimes play with friends on that world)
The game would play normally
vertical "walls" appear in random places
How to reproduce
1. Make many copy's and make copy's of copy's of a world..., delete and rename them
2. Load a reload chunks/quit game meany times
3. They might start appearing randomly
I do not know if I am doing something wrong wen coping or it is a bug.