In one game (Still ongoing, so I won't mention names), my ally successfully talked me out of a stab right when I was going for it.

In one game, i was england and had a good alliance with france, but he was focusing down south, so in 3 or 4 turns i took all 3 of his home SCs and a couple more

One time, at band camp, I stabbed russia (as italy), took 3 of his SC's in the same turn he gained 4, he builts and I successfully was able not be be utterly destroyed by the entire map.

As England, I stabbed Germany by a beautiful pair of convoys, having set up the line all the way from St.P (an army) to Belgium. Four centres in year 1, 3 in year 2 and he was down to only one centre.

But the thing was that he had become a total puppet from Spring 1901....

One time in a F2F game I was running (and playing in), one of the members of the opposing coalition stabbed 2 of his allies in the same turn and then called the third ally a traitor, AND MEANT IT. LoL...

Ah, there was a time in a F2F game, where I convinced Turkey and Austria to Ally against Italy. (I was france). Then, they attacked him, but I got tied up against England when the time came to make my move. Germany, who was helping me annihilate the English, made me stop the attack on france, in exchange for leaving Italy alone.
In the end, Italy and Germany wound up at war with Austria, Russia, and Turkey, while I successfully took the British Isle.

A successful Key Lepento. Enough said

My best ever diplomatic coup was when I tricked 7 experienced diplomacy players into telling the world their "best ever diplomatic coups" LOL!

Unfortunately, you got their best strategic coups, not their diplomatic ones.

as france, allied with germany, only turkey and russia left - couldn't break through their defensive line in the balkans (poor tactics on my part maybe) so i convinced each of them that i would help them attack the other, giving me time to reposition (while the fought ineffectually) and break through against both of them (attacking one first so the ruse continued for a turn). won the game shortly after that.