There is no specific bug report forum but since devs drop by this place sometimes, I'll leave it here.
I've encountered a nasty, though not gamebreaking bug in multiplayer games:
When human A presents a trade offer to human B and human B leaves the game before accepting or rejecting the offer, human A is unable to conduct diplomacy with other human players and the AI that took over human B.
To recreate this bug, one civ should simply send an offer to another and the second one should leave the game without answering the offer.
Game needs to be reloaded in order to make it work again.
I've encountered a nasty, though not gamebreaking bug in multiplayer games:
When human A presents a trade offer to human B and human B leaves the game before accepting or rejecting the offer, human A is unable to conduct diplomacy with other human players and the AI that took over human B.
To recreate this bug, one civ should simply send an offer to another and the second one should leave the game without answering the offer.
Game needs to be reloaded in order to make it work again.