Comment 63 for bug 587186

Well, if the preinst script *doesn't* cause a roll-back, I'd have to question if a libc6 preinst script is the 'only correct' place to prevent an upgrade. You want to leave the machine in pretty much the same state as before do-release-upgrade was called, so that, e.g. the machine can at least continue to handle Lucid updates. Otherwise the machine is just about as borked.