I am running 12.04 Precise beta (upgraded from 11.10 Oneiric) and I added the following line to my /etc/apt/source.list:

deb http://http.us.debian.org/debian unstable main contrib non-free

In order to get a newer version of a package (octave 3.6) that I needed but was not available in the precise repository.

This worked fine, but now when I want to upgrade there is a large number of packages that need to get updated. I assume this is because sid has newer versions of many of the packages than precise.

I've temporarily disabled the sid repository, and this works fine - however I am curious to know what would happen if I allowed all those upgrades to go through? Would it break my system? Are the structures of Ubuntu Precise and Debian Sid repositories fundamentally different somehow?