The code name for Debian's development distribution is "sid", aliased to "unstable". Most of the development work that is done in Debian, is uploaded to this distribution. This distribution will never get released; instead, packages from it will propagate into testing and then into a real release.
"sid" is subject to massive changes and in-place library updates. This can result in a very "unstable" system which contains packages that cannot be installed due to missing libraries, dependencies that cannot be fulfilled etc. Use it at your own risk!
If you want to run Sid, you run everything from Sid. Else you stay away, and this is by design