The only time I have ever lost was actually a non-game accident. I played a med tech, high AI, low bonus, and by year 10 I was in 2nd place with the pretorian empire in 1st place ahead of me by about 40% with a score above 2M. I was trying to play a non-aggressive game. Suddenly the game says that the victory conditions have been fufilled. I had no idea what happened. I remembered that I set the victory conditions to be peace after 10 years, and I checked the victory conditions and realized that I had accedently checked the 'after this many years elapse' box rather than the 'conditions only apply after this many years' box. Needless to say that was the first game I lost.
Scrytub Ka'Lorrith
Krintarian Imperium