floating "walls" appearing

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)

Expected result: 
The game would play normally

Actual result:
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.