Just lost a game because I didn't read my guide! Was going for a cultural win so my army was weak and my tech lagging. Brennus was a vassal of Justinian. Ramses and me were in the Apostolic Palace and good friends - I was Full Member. I had one border city with the AP religion. Neither Brennus nor Justinian were Members. What happened twice was that Justinian and vassal attacked me and captured the border city with the AP religion (making Justinian a member) - then a vote comes up and the war is ended - I later get the city back via the AP as well. The third time however, Brennus was the one that captured the city - what I forgot was that since Brennus was a vassal he is considered a non-Member for proposal conditions - so the vote to end the war never came up and I was overrun!