Dawn of War: Dark Crusade - Firestorm over Kronus 3.5 mod
A mate and I were playing an eight player map and it was set to the gametype where you can make and break alliances with anyone in the game. At the start of the battle I (as Necrons) allied with my mate who was Imperial Guard and we were fighting together for the majority of the game. Eventually the enemy AI started attacking us continually in the centre and during the confusion I sent one of my builder Scarabs into my mates base, right to the back.
I got the scarab to begin building a summoning core, my mate thought it was just so I had a base to fall back to as my original base was being attacked. However, once the enemy sent out another wave of soldiers and while my mate was distracted I teleported my entire army to the summoning core behind his base, broke the alliance with him, and proceeded to destroy his base from behind.
It was hilarious!
And the best part is that I still have the battle recorded!