Comment 2 for bug 420009

This does happen to regular Ubuntu archives as well, during the development phase. For a released version iti is obviously rare.

However, since doing an "apt-get update" can be heavy, especially on slower machines, it's not necessarily a good idea to do it always before installing upgrades. I'd say that the mechanisms we have in place of keeping the packge lists up to date daily should take of the security aspect.

It might help if the dialog saying packages couldn't be downloaded would warn about package lists being up to date. Or the upgrade action could the dates of the updates and warn if they're older than a day or two.