Just stress tested the diplomacy feature and I found out that I can trade for all the planets, techs, ships, credits, and treaty from the AI using the weekly payment feature (i.e. 999999 credit per week) and then breaking the treaty immediately. Surprisingly, I did not suffer from any repercussion for doing so. Not even a condemnation from the AI.
Secondly, by using the bug above, the AI is down to their homeplanet only. I then rushed a shipyard at the planet next to their homeplanet (i.e. Mars), then a transport, and then invade their homeplanet. The AI is therefore eliminated before the turn end. But, on the next turn, the AI will prompt for peace even though they are already eliminated. It doesn't matter whether I accept the offer or not, since a CTD will occur at some point after the prompt. I suspect that the game is still treating the AI as still alive even though they are already eliminated.
I managed to win the game though by invading all four down-to-one-planet AI at the same turn though. If I pressed continue game at the victory screen, the prompt for peace will still occur and the CTD will occur soon.