Its noticed that the update manager attempts for a "partial upgrade" (normally when the system is running beta and going to a RC or full release).
However, its removing packages which it finds are obsolete or so, without really looking for or installing a replacement package for them.
For example in my case, it removed itself. :)
In another example, it removed an important c library, which resulted in the system reinstall.
Its been discussed on ubuntu forums for some length now.
I would imagine an improved update manager removing older versions of packages only when a newer version is available for installation.
Also, it turned out that some things are different on a system when going through updates channel, as compared to a fresh install (from a CD or so) channel. Thats not really fair on the developer/enthusiast who's been following the system, only to find at the end that the system he carries is a different from the official release ?
Imagine situation of a support engineer in that case. He goes to a customer to handle prob for software A on system B. Happily assuming that he also has software A on system B (his version is through updates). It might be very much possible that he cant reproduce the issue altogether.
Again, I would imagine an improved update manager making the updated system an exact replica of the release system (as of that point in time).
Here are some threads on ubuntuforums discussing these issues ....
http://ubuntuforums.org/showthread.php?t=751775&page=12
http://ubuntuforums.org/showthread.p...66#post4694766
http://ubuntuforums.org/showthread.php?t=759107
Never happen with me during Hardy development cycle but in Gutsy I lost my /etc/apt/