How to "undo" a broken "apt-get upgrade"

[iwk <debian@vanderkleyn.com>]

Hi there,
I running stable on my production severs and testing on my development 
boxes. No problems with apt-get there. However, I running unstable on 
two desktops and one with Knoppix (which is a mixed system). And it has 
happened that on these machines an apt-get upgrade left the system 
broken. I couldn even repair it with the -f parameter. However, from the 
archives I understood that it should be possible to revert to the 
previous state. If so, how can that be arrangend?

Regards,

Iwan