it really depends on the confines of the games. For example does the game remain on the board or is it a game that takes place outside.
There is "the game" involving the board, the 34 SCs and other territories and the pieces, and there is the "metagame" which involves the conversation between players, the infinite other SCs that you will be playing in the future, buying dinner for another player, having a spousal, or intimate relation with another player so on so forth that affects the game.
As diplomacy was planned with the metagame in mind, it really is up to the compunction of hte individual player as to how badly he wants to take the metagame.
I know it is the case where in postal diplomacy or f2f, people cheat by faking orders of their opposing players, stealing sheets, placing more units than there should using sleight of hand, making other players send their sheets late so on.
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Supply Centers Ignored in Autumn
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