Gifted a territory to the opponent team. I thought that the opposing player was part of my team, but he wasn't. Just played with him in another team game recently.
Did a lot of delay moves ready for a big attack to break an opponent's bonus, then watched the turn, and realised I had delayed perfectly. Only to watch my attack hit the wasteland territory next to my opponent's territory. :( No hope of tracking down the game now, I just remember it happened.
Hmmm... I was in a big team game where I was strongest, and then I accidentally surrender, not paying attention. I had planned to clear orders, but I guess I wasn't paying enough attention.