Example from http://academia.stackexchange.com:
vs.
That happens if the answer gets deleted between the moment you load the page and click the flag link. The moment you do that, the HTTP call by JavaScript loads the flag options currently available to you. You can't do much about an already deleted answer except flagging it for moderator attention.
Sometimes, the page will indicate (through the Websocket) that the answer has been deleted (and ask you to click to reload). I'm not sure under which conditions this is supposed to happen, but apparently it didn't happen in your case.