At $DAYJOB, we're currently setting up basically a way to bridge an interface over the internet, so it transports everything that enters on an interface across the aether. Well, and you already guessed it, I accidentally configured it for eth0 and couldn't SSH in anymore.

Where it becomes fun, is that I actually was at work. I was setting it up on two raspis, which were connected to a router, everything placed right next to me. So, I figured, I'd just hook up another Ethernet cable, pick out the IP from the router's management interface and SSH in that way.
Except I couldn't reach the management interface anymore. Nothing in that network would respond.

Eventually, I saw that the router's activity lights were blinking like Christmas decoration. I'm guessing, I had built a loop and therefore something akin to a broadcast storm was overloading the router. Thankfully, the solution was then relatively straightforward, in that I had to unplug one of the raspis, SSH in via the second port, nuke our configuration and then repeat for the other raspi.