If I set the scene with remotely configuring network interfaces at 3 AM you’ll probably already guess it was a bad idea. In retrospect I would have been better served by my reservations that I should do it when I was better rested and had given it some thought. As it was, I was configuring a bridge for a KVM according to the excellent Debian Handbook.
/etc/init.d/networking reloading the network interfaces didn’t bring the new ones up, so I tried restarting them. One of the last messages I saw was "Running /etc/init.d/networking restart is deprecated because it may not enable again some interfaces". The last message I saw was a DHCP release. Whoops. What I probably should have done is run ifup on the new interfaces directly instead of risking bringing down the interface I was connected over.