I'm exeriencing the same problem and want to counter the argument that it's a
good incentive to correct the apt configuration.
I allways have backports PPA configured to have current packages in latest
(ubuntu) stable release. At the release time of a new stable, the backports
repositories are not populated (yet), but I keep them pointing to the new
version of the repo so that when developers start populating the repo I will
instantly enjoy the update without having to think about adding the repo back.
This has allways worked, until now.
Although the work-around (quit/restart aptitude) is not a big problem I just
wanted to add a (in my opinion) valid usecase of maintaining invalid
repository configuration that should preferably be ignored gracefully.