Andrew, look at the log on my comment. The error being reported is:
[20:24:11] [Server thread/ERROR]: Couldn't save chunk; already in use by another instance of Minecraft?
Basically, old chunks from one world can't be overwritten by worldgen in the next.
Blah, I'd accept "Don't do that then" as a bugfix...
Confirmed in 14w05b (OSX 10.8.5, log attached).
Just accidentally created a checkerboard of two different worlds due to the deletion of the previous world failing to complete before a new world (with the same name) was created.
I meant to create a new Creative world but created a Survival world by mistake (as "New World"). I quit out to the menu, deleted the Survival world and created a Creative one instead (also "New World"). Creative world has a mixture of chunks from the deleted world and the world that actually goes with the seed given by /seed
.
Edit: Actually, this may be an instance of MC-526 instead.
Another video, this time in Minecraft 1.11 under OS X El Capitan.