Yesterday I had approached a point in my war with China where I had them backed all the way up to their smaller tundra cities. I'm trying to win the space race, so I wanted to quit the war and get back to rushing techs. I took Nanking (Size 12) but my bombardment had taken it down to an 8. Then I stocked the victorious Infantry army, four tanks and three separate infantry, plus two workers into the city and proceeded to offer China a peace treaty, which they accepted. I assumed, since my military forces outnumbered the citizens, that the city couldn't flip. Guess what? Next turn I get the announcement...
The noob lesson? Don't offer hasty peace treaties.
The question? Are there rules to flips? Or at least signs to look out for?

The noob lesson? Don't offer hasty peace treaties.
The question? Are there rules to flips? Or at least signs to look out for?