An instance on rack2 appeared to be stuck because:

• propolis-server panicked
• the propolis-server service was restarted by SMF (its restarter is svc:/system/svc/restarter:default)
• sled-agent failed to notice that propolis-server had panicked (sled-agent failed to notice propolis-server panicked #3206), and the instance was still marked as running
• a new propolis-server ran, and requests from the serial console in the web console continued, but there was no instance, so the console seemed to hang

It isn't clear to me that restarting propolis-server is the right thing to do here. We should consider having the service go into maintenance and not restart when that happens.

Related: #2825